Add charmhelpers.contrib.openstack.ip to mocks
Add charmhelpers.contrib.openstack.ip to mocks

Reactive charms use charms_openstack.test_mocks to setup their
mocks. A recent change imports 'charmhelpers.contrib.openstack.ip'
so add that to the mock list

Change-Id: I3bd2517959b1cd43e057f420c8c4f2f28598d70a

coreycb/charms.openstack