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
Fixes OPENSTACK-362

Change-Id: If4112466b95297d4929ed21d42ce2b6acc2096ff