Neutron / Metadata: improve agent scalability

we need by default to set the metadata backlog value and the workers
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>