Remove NeutronEnableTunnelling from templates

Remove NeutronEnableTunnelling from templates

As described in https://bugs.launchpad.net/tripleo/+bug/1532830,
the OVS agent no longer uses enable_tunneling, which is controlled by
NeutronEnableTunnelling, so this change removes NeutronEnableTunnelling
from the Heat templates.

This change depends on NeutronEnableTunnelling also being removed
from python-tripleoclient and puppet-neutron no longer using the
enable_tunneling hieradata.

Change-Id: I1ff6902ebd15041fc57ffff20a07455f171a004b
Closes-Bug: 1532830
Depends-On: I28d33592374f60cb5222a866efaf9d137aca1c5a
Depends-On: I73630653330c67444827f32740c44e9d25b5db31