apstra_datacenter_generic_system - require replace when links added or removed

[chrismarget-j]

It might be convenient to destroy and recreate a generic system when links are changed.

For purposes of this issue, "changing links" means:

1. adding a new links (combination of switch and port)
2. deleting a link (combination of switch and port)

Do we need (1) here? Not clear.

The behavior should be configurable through a new attribute: link_change_requires_replace (or similar)

[chrismarget-j]

After further consideration...

Maybe only destroy the generic system when a link is removed. Adding a link doesn't trigger the sort of problematic workflow[1] this PR is intended to address.

[1] This PR is intended to handle a "shift all server links 2 ports to the right" kind of problem.

With n servers, only server n and n-1 have ports available. All others find their target port blocked by n+2.

Because terraform destroys before creating, a ResourcePlanModifier which does the destroy will make room for all n servers before any are re-created with links in recently-occupied switch interfaces.