Default value of `ovsdb_connection` does not match system configuration

[Frode Nordahl]

When creating instances you can sometimes see messages like these in the log:
2019-11-12 10:36:22.681 32728 ERROR vif_plug_ovs.ovsdb.impl_vsctl [req-159681cf-ebb2-4844-b86d-8f6e7b8d40f1 ebd65ae223c24c48b31acc8f3235927d 061f59f3ebf944918db16e7a16567074 - fa8df29bb9804ab5aff24fe38536f203 fa8df29bb9804ab5aff24fe38536f203] Unable to execute ['ovs-vsctl', '--timeout=120', '--oneline', '--format=json', '--db=tcp:127.0.0.1:6640', '--', '--may-exist', 'add-br', 'br-int', '--', 'set', 'Bridge', 'br-int', 'datapath_type=system']. Exception: Unexpected error while running command.
2019-11-12 10:37:45.904 32728 ERROR vif_plug_ovs.ovsdb.impl_vsctl [req-fce4021e-f0f7-4c25-bbd6-522ed2225bb7 ebd65ae223c24c48b31acc8f3235927d 061f59f3ebf944918db16e7a16567074 - fa8df29bb9804ab5aff24fe38536f203 fa8df29bb9804ab5aff24fe38536f203] Unable to execute ['ovs-vsctl', '--timeout=120', '--oneline', '--format=json', '--db=tcp:127.0.0.1:6640', '--', '--if-exists', 'del-port', 'br-int', 'tapb205428b-83']. Exception: Unexpected error while running command.

Open vSwitch `ovsdb-server` is not configured to listen on a TCP socket by default.

Nova's default has recently changed [0].

Perhaps we should point Nova at the Unix socket Open vSwitch `ovsdb-server` listens to?

Often instance creation succeeds, but I also see instance creation failures that correlate with the above messages.

I have yet to establish any causation between the two events.

0: https://github.com/openstack/os-vif/blob/master/releasenotes/notes/add-ovsdb-native-322fffb49c91503d.yaml

[Alex Kavanagh (ajkavanagh)]

TRIAGE: it's not clear what the bug is yet.

[Frode Nordahl (fnordahl)]

In a traditional ML2+OVS setup the Neutron OpenvSwitch agent will configure the Open vSwitch Manager listener on TCP port 6640 through the ``ovs-vsctl`` command.

With OVN this does no longer happen, the charm should add the listener.

Example command:

    ovs-vsctl --id @manager create Manager target="ptcp\:6640\:127.0.0.1" -- \
        add Open_vSwitch . manager_options @manager

[Frode Nordahl (fnordahl)]

https://github.com/openstack-charmers/charm-layer-ovn/pull/2