acl-plugin: defer the ACL plugin user module registration with ACL lookup until it...

acl-plugin: defer the ACL plugin user module registration with ACL lookup until it is needed

Registering ACL plugin user module within the "ACL as a service" infra during the plugin init
causes an unnecesary ACL heap allocation and prevents the changing of the ACL heap size
from the startup config.

Defer this registration until just before it is needed - i.e. when applying an ACL to
an interface.

Change-Id: Ied79967596b3b76d6630f136c998e59f8cdad962
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>