horizon: be more flexible in hiera neutron
horizon: be more flexible in hiera neutron

Requiring the neutron mechanism driver from hiera is too rigid, if
Neutron is not deployed in the catalog.
Be more flexible so catalog won't fail if the value is not set in Hiera.

Change-Id: I1475687c4dc53c77e763f42a440355a7c8d014bc
Partial-Bug: #1659662

openstack/puppet-tripleo,openstack/puppet-tripleo,openstack/puppet-tripleo