Set default metadata backlog to 4096

We already optimize the metadata agent by running it in multi-workers,
we need by default to set the metadata backlog value.
Inspired from 19:34: https://www.youtube.com/watch?v=AF9r_VQrcJ0
and https://review.openstack.org/#/c/95372

Change-Id: I68f3414ef56290ca27d11f8cf26febc100741d46
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Configure OVS Agent when using ML2 plugin

When running ML2 plugin, Neutron does not read the OVS plugin
configuration file, so the Agents can't work (mount tunnels, configure
L2, etc).
To make it work, you need to move the configuration in ML2 configuration
file.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Change-Id: Id70420a871edc352f80c0446e89dbcde94e5c490