Use systemd_journald receiver by default on Linux

[jeremyvisser]

Since Ops Agent effectively only ships with systemd support on Linux, I think it makes sense to change to using the systemd_journald receiver by default instead of the files receiver.

The deb/rpm packages only ship with systemd unit files, not sysvinit scripts. RHEL 7+ requires systemd, Debian Jessie and above have started to remove sysvinit scripts from packages. It's a reasonable expectation that ~everyone using Ops Agent on Linux will be doing so on systemd with journald.

Using the systemd_journald receiver gives significantly richer log fields, which makes filtering and searching in Cloud Logging much more convenient.

So I propose this part of the default config from...

logging:
  receivers:
    syslog:
      type: files
      include_paths:
      - /var/log/messages
      - /var/log/syslog

...to:

logging:
  receivers:
    syslog:
      type: systemd_journald

[igorpeshansky]

This will break backward compatibility, so we should consider this for the next major version. @qingling128 FYI.

[qingling128]

Created an internal tracker: b/244786954