Remove default value for CONFIG_QUANTUM_OVS_BRIDGE_MAPPINGS

The CONFIG_QUANTUM_OVS_BRIDGE_MAPPINGS had a default value of
physnet1:1000:2000, which is a vlan range, not a valid bridge mapping.

Change-Id: Id2d3a5c99a8bea2c41fa2517b7aaaaa35465794b
Merge "Remove default value for CONFIG_QUANTUM_OVS_BRIDGE_MAPPINGS"