Add EMS provider_region column; Undo hack for EmsAmazon to use hostname.

In addition, this will enable us to support OpenStack regions in an analogous way to how we support Amazon regions.

@blomquisg @gmcculloug Please review
@dclarizio There is a small UI change here, so please review that one

Not sure if there is an open RFE for OpenStack regions...still looking for that.
Should we hide hostname here?  I.e., replace hostname with provider_region .. would always be `nil` anyway, right?