Sync install_venv_common from oslo-incubator

Sync install_venv_common from oslo-incubator

The install_venv_common modules hasn't had a sync done
since early in the icehouse release.  This sync brings
Cinder's version up to date with the latest code.

Current HEAD in OSLO:
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commit 36b0e8570b449129d6d474c03b02ceb62edb78df
Date:   Thu Dec 11 11:27:08 2014 +0100
We shouldn't replace `oslo-incubator` in comments

Change being merged with this patch:
---------------------
fe3389e5 - Improve help strings

Change-Id: I66fd66650a7106b8d20e4fc483dabb91184922f9