Make --vlan option work in network-create in VLAN mode

When creating a network with network-create, DuplicateVlan exception
happens whatever the --vlan option is set. This is because --vlan
option isn't read correctly by novaclient. This patch fixes this
bug.

Fixes bug 1167779

Change-Id: I3bf0e8d96d995632698f5aa1b1a07ead9e553c70