The OVS bridge creation logic for OVN deployments may fail
when the provider bridge has not been defined. This patch uses
logic that exists in the OVS deployment scenario to check the
length of neutron_provider_networks.network_mappings to ensure
a value has been set before attempting to create the bridge.
Change-Id: I34256e4ad22169ae6907a3c40270cb714cf33466
(cherry picked from commit 7c0ac931b0accd02f4c03b994dcaaa65a2b901ae)