Import install_venv from oslo

The new version of install_venv_common evaluates requirements.txt
and test-requirements.txt at the same time and we avoid Django 1.6
is installed when evaluating django-nose dependencies.

Change-Id: I2ab7108b3615ac6c0525089b81b80e5c0a4793fc
Closes-Bug: #1250581
Remove docs for removed can_encrypt_volumes setting

Change I730b4ffc26f391065943cb0d135ceac113cb6754 removed
support for encrypted volumes, but didn't remove the docs.

Change-Id: I29854e9d0abcccbfe3949797908016780b47ff6c
Closes-bug: #1250711
Merge "Remove docs for removed can_encrypt_volumes setting"
Merge "Import install_venv from oslo"