Fix issues if ML2 neutron is configured with linuxbridge

+  For linuxbridge, the external_network_bridge option should be empty.
   See: https://github.com/openstack/neutron/blob/master/etc/l3_agent.ini#L43
   We should allow CONFIG_NEUTRON_L3_EXT_BRIDGE to have an empty value.

+  The example given in usage for NEUTRON_LB_PLUGIN_AND_AGENT is false.
   The linuxbridge need to have a mapping between a network and interface,
   not a network and a bridge. See: https://github.com/openstack/neutron/
   blob/master/etc/neutron/plugins/linuxbridge/linuxbridge_conf.ini#L23

+  create_l3_manifests try to read CONFIG_NEUTRON_OVS_BRIDGE_MAPPING
   for all types of bridge. This patch fix the issue by moving the
   'if openvswitch' test.

Closes-Bug: #1399733
Change-Id: Ia7297c9941d93f822ed4b7ea4f4bf4932304ac64
(cherry picked from commit 8138aee5dc21bc12ee6b62a23e9d1ef52176b7ff)
Merge "Fix issues if ML2 neutron is configured with linuxbridge" into juno