Re-arranged generation of provider network options

There should be no difference between provider networks generated for
linuxbridge or openvswitch setups, when using the "VLAN" networking modes.

Previously we didn't add any --provider options in openvswitch based setups.
Which cause the segmentation_id attribute to be auto-populated with a wrong
value.
Honor the :use_vlan flag when creating the floating network

If use_vlan is false for the floating network we need to create the floating
network with type "flat". Otherwise it will endup being tagged to a VLAN and be
inaccessible from the public network (especially when using linuxbridges).