Add -U to the command line for pip

Since we have site-packages=True in nova, the existence of system
packages of the wrong version can cause nova to not install what
you expect into the tox virtualenv. Adding -U into the deps list
causes it to get appended into the command line, which causes pip
to do what we want and install what we asked for.

Change-Id: I4f388e07caf31ab870f22d261e856a205ebb36c0
Merge "Add -U to the command line for pip"