Fix config of ML2 plugin in neutron element

tenant_network_types option of ML2 neutron core plugin
defaults to 'local' (i.e. no communications beyond one
physical machine). We must use 'gre' tenant networks
instead, just like we did for OVS neutron plugin.

Closes-Bug: #1251166

Change-Id: I3779412ac6ee2d7291a8a4dc52c7e1fea79a54b7
Merge "Fix config of ML2 plugin in neutron element"