Prepare 1.6.0 release of bootstrap. Update docs

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Release 1.6.0.

Changelog

• (#254) Add a --force-content-source option. Useful when you have remote execution and hosts that may not have subnets defined.
• (#252) Add a user-friendly comment
• (#250) Support Arbitrary servers as Puppet Master and Puppet CA, useful for load-balanced Puppet Masters.
• (#249) bootstrap.py now verifies if the domain that a host is using is associated with the correct org/location.
• (#246) adds a --content-only option as an alias to --skip foreman
• (#243) Allows a user to specify Location -L when using either --content-only or --skip foreman
• (#244) Documentation updates
• (#245) bootstrap.py now runs a yum clean all when unregistering from another server
• (#241) Updating the documentation for various --skip options.
• (#216) implement pylint checking.