Allows the definition of the global variable OVS_BRIDGE_MAPPINGS (e.g.
in local.conf) to automatically trigger the creation of multiple OVS
bridges. For example:
OVS_BRIDGE_MAPPINGS=physnet1:br-br-enp0s20f1,physnet2:br-enp0s20f2
should automatically yield the creation of two bridges, respectively
associated to the two physical networks declared,
by simply running DevStack with the OVS agent enabled.
Documentation has also been added to doc/source/guides/neutron.rst.
Change-Id: I79dc0213c9d70ba628621c4c0f65481783590085
Closes-Bug: #1535835
3095ff5132