SAIC-167: Fix bug with nova security groups
SAIC-167: Fix bug with nova security groups

Sometimes in get_instance response key "security_groups" is absent.
This patch makes safe receiving of security_groups with nova's response.

andybondar/CloudFerry,SVilgelm/CloudFerry,ntymtsiv/CloudFerry,Settis/CloudFerry,mgrygoriev/CloudFerry,andybondar/CloudFerry,MirantisWorkloadMobility/CloudFerry,roman-verchikov/CloudFerry,archyufa/CloudFerry,roman-verchikov/CloudFerry,mgrygoriev/CloudFerry,MirantisWorkloadMobility/CloudFerry,ntymtsiv/CloudFerry,MirantisWorkloadMobility/CloudFerry,andybondar/CloudFerry,japaniel/CloudFerry,roman-verchikov/CloudFerry,archyufa/CloudFerry,realsystem/CloudFerry,SVilgelm/CloudFerry,mgrygoriev/CloudFerry,andybondar/CloudFerry,archyufa/CloudFerry,archyufa/CloudFerry,japaniel/CloudFerry,SVilgelm/CloudFerry,ntymtsiv/CloudFerry,japaniel/CloudFerry,realsystem/CloudFerry,ntymtsiv/CloudFerry,realsystem/CloudFerry,roman-verchikov/CloudFerry,SVilgelm/CloudFerry,realsystem/CloudFerry,mgrygoriev/CloudFerry,MirantisWorkloadMobility/CloudFerry,japaniel/CloudFerry