Add multi-region support

If only one region is available then the Neutron CLI will just pick the
only one available.  However, if more than one is available, then it
will error out about ambigous region.  This patch extends the base
provider code to parse out the nova_region_name when it's parsing out
the keystone credentials and define the OS_REGION_NAME environment
variable when calling neutron, if the nova_region_name variable exists
in the config file.