Changes:
Fixed as described in the bug.
* CreateVlanIpRangeCmd still accept account/domainId info
* if account owns:
- one Isolated network with source nat service enabled, use this network
- more than one Isolated network with source nat service enabled - error out
- none Isolated networks with source nat service enabled, create it only in
case when there is an Isolated network offering with Availability=Required and
source nat service enabled.