Enhance run() placeholder substitutions to honor configuration defaults

[mih]

Previously, run() would not recognize configuration defaults for placeholder substitution. This means that any placeholders globally declared in datalad.interface.common_cfg, or via register_config() in DataLad extensions would not be effective.

This changeset makes run's format_command() helper include such defaults explicitly, and thereby enable the global declaration of substitution defaults.

Moreoever a {python} placeholder is now defined via this mechanism, and points to the value of sys.executable by default. This particular placeholder was found to be valueable for improving the portability of run-recording across (specific) Python versions, or across different (virtual) environments. See
datalad/datalad-container#224 for an example use case.

A corresponding test is included.

The ability to keep run-records paramterizable, in particular, for interpreters can also aid longevity (think platform-specific choices to call a system Python executable python3 for some years, and then switch back.

Related datalad/datalad-container#250

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (a96c51c) 91.56% compared to head (776e66b) 91.56%.
Report is 47 commits behind head on maint.

❗ Current head 776e66b differs from pull request most recent head 7b007cd. Consider uploading reports for the commit 7b007cd to get more accurate results

Files	Patch %	Lines
datalad/core/local/tests/test_run.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            maint    #7509      +/-   ##
==========================================
- Coverage   91.56%   91.56%   -0.01%     
==========================================
  Files         325      325              
  Lines       43443    43457      +14     
  Branches     5827     5828       +1     
==========================================
+ Hits        39780    39791      +11     
- Misses       3648     3651       +3     
  Partials       15       15              

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[mih]

MacOS failure looks unrelated (web remote).

[yarikoptic]

I need to understand it better: isn't this just a singular case of a larger common (?) pattern that in many (most?) cases we would actually like something like this for any Dataset.config access since dataset's config should be a "union" with system/user/env-overloads settings?

[mih]

I dont know if this is a common case. Typically, access that requires acknowledging a configurable default goes via obtain(). Here it is different, because it needs to look for matches and not for a specific item.

[yarikoptic]

appveyor might have stalled (restarted).

[gw1] PASSED ../datalad/support/tests/test_sshconnector.py::test_bundle_invariance 
../datalad/support/tests/test_sshrun.py::test_exit_code CommandError: 'ssh -o ControlPath=/home/appveyor/.cache/datalad/sockets/bad871d9 datalad-test 'exit 42'' failed with exitcode 42
[gw1] XPASS ../datalad/support/tests/test_sshrun.py::test_exit_code 
../datalad/support/tests/test_sshrun.py::test_no_stdin_swallow 
[gw1] PASSED ../datalad/support/tests/test_sshrun.py::test_no_stdin_swallow 
../datalad/support/tests/test_sshrun.py::test_fancy_quotes 
[gw1] PASSED ../datalad/support/tests/test_sshrun.py::test_fancy_quotes 
../datalad/support/tests/test_sshrun.py::test_ssh_option 
[gw1] SKIPPED ../datalad/support/tests/test_sshrun.py::test_ssh_option 
../datalad/support/tests/test_sshrun.py::test_ssh_ipv4_6_incompatible 
[gw1] PASSED ../datalad/support/tests/test_sshrun.py::test_ssh_ipv4_6_incompatible 
../datalad/support/tests/test_sshrun.py::test_ssh_ipv4_6 ssh: Could not resolve hostname 127.0.0.1: Address family for hostname not supported
[gw1] PASSED ../datalad/support/tests/test_sshrun.py::test_ssh_ipv4_6 
../datalad/support/tests/test_stats.py::test_ActivityStats_basic 
[gw1] PASSED ../datalad/support/tests/test_stats.py::test_ActivityStats_basic 
../datalad/support/tests/test_stats.py::test_ActivityStats_comparisons 
[gw1] PASSED ../datalad/support/tests/test_stats.py::test_ActivityStats_comparisons 
../datalad/support/tests/test_stats.py::test_add 
[gw1] PASSED ../datalad/support/tests/test_stats.py::test_add 
../datalad/support/tests/test_status.py::test_FileStatus_basic 
[gw1] PASSED ../datalad/support/tests/test_status.py::test_FileStatus_basic 
../datalad/support/tests/test_vcr_.py::test_use_cassette_if_no_vcr 

OSX restarted... travis seems no more (I ended subscription but was hoping that opensource thing would kick in... will check later)