Issue title: virtualenv does not use SCL path from environment
 Issue body: We have to use a alternative python2.7 installation via RedHat SCL. Is there a possibility to specify the path of the python executable for python38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5virtualenv? virtualenv.py uses the PATH env variable to discover the python binary. Unfortunately exported paths via Exec do not affect the PATH within the puppet run.

 Comments: 
 Comment 0: The following `path`-option provided by `python38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5virtualenv` solved my problem. First just looked at the readme which is incomplete, completing the README.md would be helpful.

###### path
* Specifies the PATH variable. Default: [ '/bin', '/usr/bin', '/usr/sbin' ]
 Comment 1: @seblau done https://github.com/stankevich/puppet-python/commit/48202f1690e40d1d673373b25c72a193ee1d5482

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