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
fix help text in "Create An image" window

In the "Create An image" window, there are "minimum disk" and "minimun
memory" text input. But both help text on the two text inputs are using
"the minimum disk size...". Actually for the second one we should say
"the minimum memory size...", and now this patch fix this.

Change-Id: Ife19acba5fafb41d99e9416d2f9e8082b9eaef53
Merge "Remove docs for removed can_encrypt_volumes setting"
Merge "Import install_venv from oslo"
Merge "fix help text in "Create An image" window"