Check DB scheme prior to migration to Ml2

When using migration tool from LB/OVS plugin to Ml2, there is no
guarantee current scheme is supported by migration tool. This patch
checks version stored in DB by alembic and compares whether version
is supported.

Closes-bug: #1307720

Change-Id: I4519a0e5a0f3027675958a68d1f9e0440b177229
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 "Check DB scheme prior to migration to Ml2"
Merge "Start an unstarted patch in the hyperv unit tests"
Merge "Add an option to turn off DF for GRE and VXLAN tunnels"