(PUP-3236) Support PUPPET_LOADED env var in Gemfile

This commit adds support for an environment variable, PUPPET_LOADED,
to be set during bundle installation.  If set, the variable will
prevent the puppet gem from being loaded; the use case is for test
environments where you already have puppet available on the load path.
Merge pull request #3086 from cprice404/maint/stable/PUP-3236-gemfile-puppet-loaded

(PUP-3236) Support PUPPET_LOADED env var in Gemfile