Add requirements.txt support to os-svc-install.

http://pad.lv/1179008 has all of OpenStack migrating tools/pip-requires
to requirements.txt. We will use both during the transition.

Fixes bug #1179008

Change-Id: I7b3c64902b4c1b6f83402a21248a4eadad725c34
Merge "Add requirements.txt support to os-svc-install."