Add sync point between head node and login nodes + make DFSM check able to handle missing change-set

[gmarciani]

Description of changes

1. Add a syncronization point between head node and login nodes, so that login nodes wait for the head node to write the keys before attempting to import them. The lack of dependency is the source of sporadic bootstrap failures in login nodes.
2. Make DFSM check able to handle missing change-set: when a change-set file is missing then for sure mount/unmount is not required.
3. Clarify a comment describing the role of a syncronization file
4. Add tests

Tests

• Spec test
• Kitchen: ./kitchen.ec2.sh environment-config test login-nodes-keys-configuration-alinux2
• test_shared_home succeeded; it used to show login nodes bootstrap failures before this change.

References

• Link to impacted open issues.
• Link to related PRs in other packages (i.e. cookbook, node).
• Link to documentation useful to understand the changes.

Checklist

• Make sure you are pointing to the right branch.
• If you're creating a patch for a branch other than develop add the branch name as prefix in the PR title (e.g. [release-3.6]).
• Check all commits' messages are clear, describing what and why vs how.
• Make sure to have added unit tests or integration tests to cover the new/modified code.
• Check if documentation is impacted by this change.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.48%. Comparing base (77832de) to head (dbc7af3).

Additional details and impacted files

@@             Coverage Diff              @@
##           release-3.9    #2671   +/-   ##
============================================
  Coverage        76.48%   76.48%           
============================================
  Files               22       22           
  Lines             2220     2220           
============================================
  Hits              1698     1698           
  Misses             522      522           

Flag	Coverage Δ
unittests	76.48% <ø> (ø)

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