Add namespace cleanup options

Adds a configuration option to tell the network agents to delete
namespaces when they are no longer in use. The option defaults to
False so that the agent will not attempt to delete namespaces in
environments where this is not safe.

It affects only L3 & DHCP agents for now.

Change-Id: Ie9e27ae55b796ec12601dbfbeda45184188864f5
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>