Introduce a ifmap dependency manager

This patch is a part of the SNAT
and will be leveraged by the service
instance class that is going to be introduced
in a next patch.

Change-Id: I22c61f9b1de5778285327664572a88f597470060
Authored-by: Pedro Marques <roque@juniper.net>
Co-Authored-By: Sylvain Afchain <sylvain.afchain@enovance.com>