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
make zone id optional parameter in createVpc, zone id can be null only
if vpc offfering supports region level VPC
in region level vpc, let the network/tier to be created in any zone of
the region