CLOUDSTACK-4294 After upgrade of CloudStack to 4.2, support old vswitch type in existing clusters if user changes the vswitch backend to another type of vSwitch

While upgrading to 4.2, the type of vswitch being used by each cluster is persisted to cluster_details table. This helps if user want to change the type of vswitch used in a zone or entire cloud later on but leave existing cluster continue to use old vswitches. Hence even after modifying the type of vswitch at cloud level (by modifying global configuration parameters) or modifying the type vswitch at zone level (by modifying the traffic label) would not disturb operation of existing clusters.

Signed-off-by: Sateesh Chodapuneedi <sateesh@apache.org>