Fix All the Things

This PR cleans up most open issues. Highlights include:

* Update to recent Berks, Test Kitchen and Rubocop.
* Document that `jenkins_ssh_slave` should only be used on the master.
* Only create user, group and directories on war installations.
* Ensure compiled attributes respect overrides.
* Ensure plugin installs respect global mirror setting.
* Lots of integration test improvements:
  * Split `jnlp` and `ssh` slave fixtures.
  * Tests should use WAR install locked to 1.555.
  * Add fallback to `jenkins_slave` matcher if authn is enabled.
  * Update authn int tests to load private key from data bag item.
  * Add integration test coverage for smoke suites.
  * Add int test coverage for ssh slaves with password creds.
  * Use a mirror for integration testing.

Fixes #159, #162, #163, #181.

/cc @opscode-cookbooks/release-engineers