introduce 'RegionLevelVpc' as capability of 'Connectivity' service. Add
support for CreateVPCOffering to take the 'regionlevelvpc' as capability
of service 'connectivity'.
introduces new capability 'StretchedL2Subnet' for 'Connectivity'
service. Also add support to createNetworkOffering api to allow
StretchedL2Subnet capablity for the connectivity service.
adds check to ensure 'Connectivity' service provider supports
'StretchedL2Subnet' and 'RegionLevelVpc' capabilities when specified in
createNetworkOffering and createVpcOffering respectivley
enable ovs plug-in to support both StretchedL2Subnet and RegionLevelVpc
capabilities