Add support for provider networks

Allow setting CONFIG_QUANTUM_L3_EXT_BRIDGE=provider to use a provider
network instead of creating the br-ex bridge. Also, add a new config
option, CONFIG_QUANTUM_OVS_BRIDGE_IFACES, to allow adding network
interfaces to OVS bridges. For example:

CONFIG_QUANTUM_OVS_VLAN_RANGES=physnet1:1000:2000
CONFIG_QUANTUM_OVS_BRIDGE_MAPPINGS=physnet1:br-eth1
CONFIG_QUANTUM_OVS_BRIDGE_IFACES=br-eth1:eth1

would set up a network labeled 'physnet1' with an OVS bridge named
br-eth1, and add the eth1 interface to the br-eth1 OVS bridge.

Change-Id: I9c9f4c22b15ab740dc09ede2d33c9499c0105732