openvswitch/ofagent: Remove OVS.enable_tunneling option

The option has been marked deprecated in IceHouse.
Please use AGENT.tunnel_types=gre instead.

DocImpact
Related-Bug: #1195374
Change-Id: Iea6159c5949a63a0b680818a3fd0928d470115bd
Merge "openvswitch/ofagent: Remove OVS.enable_tunneling option"