Add special cases for Chef and Ruby

* Add /opt/chef/embedded/bin to PATH if it exists
* Set ENV_NO_RVM to turn off rvm if the Chef bin directory is found,
  since rvm and Chef hate co-existing and have restraining orders
  against the other
* Track rbenv usage with ENV_USE_RBENV
* Add rvm bin directory to path and run the init script ONLY if
  ENV_NO_RVM and ENV_USE_RBENV are not set