While OVS was used initially due to availability of a
networking-generic-switch driver for it, Linux bridge can be useful for
environments where OVS is not available.
This is configured via bridge_type variable. It defaults to
'openvswitch', but may be set to 'linuxbridge'.
Change-Id: I750a73cebc743edcbcd8c23c67e4920d0058bead
ced8a4a528