Use in-tree openvswitch datapath module if the kernel is new enough

The major part of the openvswitch datapath kernel module has been merged into
Linux 3.13. In particular both GRE and VXLAN tunnels are supported (which are
used by neutron). The out-of-tree module is no longer actively developed.
In particular, version 1.10 and 1.11 do not support kernel newer than 3.8
(although Debian maintainers made them work with kernels <= 3.11).

Therefore use the out-of-tree module only with old kernels (the ones older
than 3.13)

Change-Id: Ic0f19bb451cb6a27ba4f17431810301414e711c3
Related-bug: #1390044
Related-bug: #1383641