Auto-merge could merge unapplied infra changes

[parmouraly]

Version: 0.11.1

Issue
When auto-merge is enabled, it can happen that Atlantis will merge a PR with code that would normally result to infrastructure changes, but without ensuring these changes are applied.
This means you end up with drift between your infrastructure state and the code representing it in your master branch.

Expected behaviour
Even with auto-merge enabled, Atlantis should never decide to merge a PR with code that would normally generate a plan diff which has not been successfully applied

Example/reproduce

[parmouraly]

This could be a fix

#1005