Parameterize network type and tunnel types

Remove the hardcoding of gre as the Neutron tenant network type for the
Overcloud. This will enable the ability to deploy an Overcloud that uses
vxlan instead of gre tunnels. A new parameter, NeutronTunnelTypes, is
added to allow configuring the tunnel_types parameter in the Neutron ML2
configuration.

This change is required by https://review.openstack.org/#/c/92913

Change-Id: I2c2e2153a61349e58ada28c87aa2338c9f00e7bd
Merge "Parameterize network type and tunnel types"