Disable "use_vlan" for the nova_fixed network

This is the first step to fix the vlan mode for quantum when using the
openvswitch driver.

"nova_fixed" is now only used for quantum when using the vlan mode for ovs or
linux-brigde.

When ovs is used in vlan-mode quantums sets up an ovs-bridge to do the vlan
tagging. That bridge needs be attached to none VLAN interface in order to work
correctly.

When linux-bridge is used quantum's linux bridge agent will create the correct
vlan interfaces automatically.