core: add keystone url field to external providers

[RFE] Add keystone URL for OpenStack external providers that require
authentication.

Currently, all OpenStack external providers like Neutron and Glance are
using the same Keystone authentication URL stored in the KeystoneAuthUrl
configuration value. The requirement is to add a URL field per such
provider for setting that URL and enable different OpenStack providers
using different Keystone authentication URLs

Change-Id: Ib1aa930cd8dec576a6408402dd883ab5162e1f9d
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1157999
Signed-off-by: Eli Mesika <emesika@redhat.com>