Permit specifying ovs bridge mappings too.

Another minor change to get a fully automated ci-overcloud deployment.

It's a shame this is all environment variable based - we'll have to
circle back and make this much nicer!

Change-Id: I5715a0ad6931c05dc10093c9abd5ecdb908e8246
Make the overcloud-user config identify itself

This just avoids confusion when using its credentials.

Change-Id: Ic2131a4df551e43140b5d8ae35041432104ca07e