[WIP] Ems refresh status

Capture the status of an EMS Refresh (success or failure).

Currently captures the status for Openstack.  Need to look at adding this either the refresher super class or to each individual provider's refresher.
Not sure we really need two date columns, though I see you are using it for logging.  Don't we really just need `:last_refreshed_on` and the status itself would determine if its valid or invalid?