Add unit test to test running from command line

This test effectivly runs all of the python code ran by
packstack --install-hosts=127.0.0.1 --os-swift-install=y \
--nagios-install=y

It is a fairly wide net but boost code coverage of the packstack
python code to about 85%, more finer grained tests should also be
Added to target speficic test cases.

Popen is replaced in PackstackTestCaseMixin so no actual commands get
run on the host running the unit tests

Change-Id: Ie5208394d6eb6034a3044014ffc35482805e41fd