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