Disable DF bit for tunneled installations

Due to the way how OVS handles GRE tunnels
it tries to copy DF flag to wrapping GRE
packet header, thus making it impossible
for packets of more than 1430 bytes size
to be transfered through regular interfaces
this breaks almost all applications. Thus
we are setting dont_fragment option to false
and we will also need to make ovs agent
set df_inherit flag to this option for
OVS versions that support df_inherit flag

We will also need to document that user should
extend his MTU size to reasonable one according
to http://docs.openstack.org/icehouse/install-guide/install/yum/content/neutron-ml2-network-node.html
in order to not get performance issues.

DocImpact
Change-Id: I031351b36f56881f21986b412bfae79fba0e884b
Partial-bug: #1402046