show diff between pip installed packages and requirements

This change will display the difference between which python packages pip
installed and the packages from the requirements lists.  It will print
the list of dependencies to the console (not including the directly declared
dependencies from the requirements list).

Closes-Bug: #1172417

Change-Id: I9363b50d6d7f5ef19c42bb2e30de45e352060ccd
Use wheels for installation

Add an option to pbr to use wheels when installing things from pip. The
speed boost this gives is quite sexy, as it can avoid having to
re-build C things all the time.

Change-Id: Id9b9b21ef949ff44c7c962e81576266661264a13