[CF-475] Add handling oh HTTP 504 to glance.
[CF-475] Add handling oh HTTP 504 to glance.

Add handling of HTTP 504 to glance migration code.

SVilgelm/CloudFerry,SVilgelm/CloudFerry,MirantisWorkloadMobility/CloudFerry,SVilgelm/CloudFerry,MirantisWorkloadMobility/CloudFerry,MirantisWorkloadMobility/CloudFerry,MirantisWorkloadMobility/CloudFerry,SVilgelm/CloudFerry