With this small changes it becomes possible to instruct murano to
provision murano-agent of version other than latest, or from
custom git branch or from a http source.
Use cases:
* Use agent with custom modifications that are not available in
PyPI version
* Development of new agent features - agent can be installed from
the private git repo
* Environments without internet connectivity
Change-Id: Icbea95abf070ef35781474a54461cc34bb9927af
ef3b0d4f7d