Fix network creation in Vlan mode

- If "vlan" is specified when creating a network, we honour the option.
- If it is not specified, the Vlan number is calculated as follows:
CONF.vlan_start + num_existing_networks + index_of_requested_network
- If the above vlan is in use, try to get the next free vlan.

Fixes bug 1194835
Fixes bug 1169026

Change-Id: Id4476c14f9dadcf498a499534337477b3f64f604
Merge "Fix network creation in Vlan mode"