Fixes the ability to deploy a venv in cases where:
1) developer_mode is not enabled
2) A cached venv is not downloaded from the repo server
Also introduced is a var that can be used to disable the cached venv
download attempt entirely and force the fallback behavior. Previously
that would have been available by simply setting the venv download
to some broken URL like "http://localhost/none", but
keystone_venv_download will allow for much easier disabling of the
cached venv download behavior.
Change-Id: Ifb07eb2a73cc3478a36177d53a9a2b0579da4e12