Add ability to pull python-swauth from git repository

Distribution packaged versions of python-swauth can be very old.
Installing python-swauth-1.0.8 from gholt's GitHub repository
brings in the ability to set allow_overrides so that middleware
such as TempURL and FormPost work well with swauth

* Added attributes to allow toggling between distribution package / git
* Added attributes to specify repository and tagged revision
* Added conditional install based on above attributes
* Bumped metadata and CHANGELOG

Change-Id: I5eaafb8af9977939bbb7f5b68baf3c9cbac8be7c