feat(cluster): rabbitmq clustering support

[noelmcloughlin]

PR progress checklist (to be filled in by reviewers)

• Changes to documentation are appropriate (or tick if not required)
• Changes to tests are appropriate (or tick if not required)
• Reviews completed

---

What type of PR is this?

Primary type

• [build] Changes related to the build system
• [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ci] Changes to the continuous integration configuration
• [feat] A new feature
• [fix] A bug fix
• [perf] A code change that improves performance
• [refactor] A code change that neither fixes a bug nor adds a feature
• [revert] A change used to revert a previous commit
• [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [docs] Documentation changes
• [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No.

Related issues and/or pull requests

Describe the changes you're proposing

This PR introduces a working solution for clustering support by managing "erlang cookie" and providing better documentation on clustering in the README and pillar.example files. Verified on CentOS7.
Some minor "bugs" fixed.

Pillar / config required to test the proposed changes

See "cluster" section in pillar.example.

Debug log showing how the proposed changes work

Result of running rabbitmq.cofig.cluster state on a member named host2 to join cluster on host1.

ID: rabbitmq-config-cluster-rabbit@locahost-join-host1
    Function: rabbitmq_cluster.joined
      Result: True
     Comment: Clustering node rabbit@host2 with rabbit@host1
     Started: 23:16:56.862987
    Duration: 11726.457 ms
     Changes:
              ----------
              new:
                  rabbit@host1
              old:
Summary for local
-------------
Succeeded: 10 (changed=1)
Failed:     0

Documentation checklist

• Updated the README (e.g. Available states).
• Updated pillar.example.

Testing checklist

• Included in Kitchen (i.e. under state_top).
• Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• Updated the relevant test pillar.

Additional context

[noelmcloughlin]

Selfie merging. Thanks, @myii for fixing _metadata issue.

[saltstack-formulas-travis]

🎉 This PR is included in version 1.1.0 🎉

The release is available on GitHub release

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