Deprecate enable_tunneling in the OVS agent

This patch properly deprecates the 'enable_tunneling' option in the OVS agent.
It does this be ignoring it if 'tunnel_types' is set, and if 'enable_tunneling'
is set, it defaults 'tunnel_types' to only GRE.

Fixes bug 1195374

Change-Id: I27fe6b930541a514fc51d3a6669347e52db8cb46
Merge "Deprecate enable_tunneling in the OVS agent"