[Openstack][provisioning] use Security Group ems_ref instead of name

Openstack allows you to create multiple security groups with the same name
If you attempt provisioning by name and there are multiple matches,
you will will receive an API error (HTTP 409 or 500).  Using the ems_ref
is guaranteed unique.  We display the name and description in the dialog,
so no further changes are required

https://bugzilla.redhat.com/show_bug.cgi?id=1161322