vagrant: switch from keystone to swauth

We use swauth for our Zebra ZeroCloud deployment, so this brings the dev
environment closer to our canonical production configuration.

I also managed to figure out how to do all of the swift configuration
changes during ./stack.sh, so no devstack restart is required before
everything works--just run `vagrant up` and you're ready to go.
Merge pull request #168 from larsbutler/vagrant-swauth

vagrant: switch from keystone to swauth