VMware: Fix SwitchNotFound error when network exists

In FlatNetworking mode, when a virtual network exists on the vCenter
host we still check for the presence of a physical adapter specified
in vlan_interface. This check is only needed if we are going to
create a new network on the vswitch associated with the interface.

This fix removes the check for the physical adapter and the vswitch
when network already exists.

Fixes: bug #1226238
Change-Id: I1eecf2c339534d8694ec6a8bdfaff5217ba2f8ef
Merge "VMware: Fix SwitchNotFound error when network exists"