Install ipset for the ml2 agent

Neutron recently added a new enable_ipset option which
helps optimize applying large iptables rules via the use
of ipset.

This patch installs the ipset package which is now required
because enable_ipset=True is the default.

Change-Id: I6078fa5e8f0f8639bcd8964765a8fa322bb7b3b1
Related-bug: #1369381
Related-bug: #1369386
Merge "Install ipset for the ml2 agent"