Add OVN Controller Gateway agent in neutron_bind_l2_agent_types

Add OVN Controller Gateway agent in neutron_bind_l2_agent_types

It avoids blacklisting NeutronNetworks.create_and_bind_ports in case of
OVN. It conforms with the upstream testing [1] and then reverts the
previous change [2].

[1] https://github.com/openstack/networking-ovn/blob/master/zuul.d/networking-ovn-jobs.yaml#L299
[2] https://git.opnfv.org/functest/commit/?id=8e297c49c481961959b29c2080cc059fcc84e9ee

This reverts commit 8e297c49c481961959b29c2080cc059fcc84e9ee.

Change-Id: I95f1b7146e636ae3ebe9f995733e53ad1b276d26
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>