Neutron / Metadata: 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

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