Use only the lower 16 bits of iptables mark for marking

Use only the lower 16 bits of iptables mark for marking

Since a packet can only have one mark, and we will need to mark a
packet for multiple purposes, we need to use a coordinated bitmask for
the two cases of simple marking that we currently do in Neutron
leaving the other bits for address scopes.

DocImpact

Change-Id: Id0517758d06e036a36dc8b8772e41af55d986b4e
Partially-Implements: blueprint address-scopes