Something appears to have caused the xenial guests to install all
python libraries into python 3.5 directories and therefore the guest
won't launch. This change forces pip2 and in theory should get around
that.
Moved Redis and PostgreSQL installs after the trove-dep ones.
Change-Id: I3bbe3bafa7ea3e627272103ac16a38f6a32a8a06
Partial-Bug: #1650382