Adding path_prefix to run_command so that one can pass in a path to the run environment if you nees something in a non-default path.
Adding the bin/ directory of the virtualenv (if you specify a virtualenv) as a path_prefix so that other programs that only exist in the virtualenv will be available to things being installed into said virtualenv.  Classic example: installing gevent requires cython binary to be available, but if cython is in the virtualenv only it won't be found without this.
Merge pull request #4413 from jerm/ansible

Add capability to pass in a PATH prefix to run_command and allow pip module
to utilize that to make virtualenv bin/ available in pip installs