Add an option to turn off DF for GRE and VXLAN tunnels

Modifications included allow to set a new option (dont_fragment) in
the ovs agent configuration file that can be used for (un-)setting the DF
bit on GRE or VXLAN tunnels. The default behaviour is not altered (DF on).

Change-Id: I17ecb00165990b72ab121c2688097139b3f2f157
Implements: blueprint neutron-ovs-agent-df-gre-vxlan
Start an unstarted patch in the hyperv unit tests

This starts a patch that was setup but never started
for a loopingcall that was allowing an occasional
exception to be thrown from the agent on unrelated patches.

Closes-Bug: #1327974
Change-Id: I6cfdb128c2a78e9c087d79a6db850603f2d03fa8
Merge "Start an unstarted patch in the hyperv unit tests"
Merge "Add an option to turn off DF for GRE and VXLAN tunnels"