Update docs for building ovs-dpdk images
Update docs for building ovs-dpdk images

Correcting and clarifying the documention for how to build OVS images
with DPDK support:

 * The additional heading was confusing and the following sentence even
   more so.
 * Fixed the image name in the build command.
 * Updated the OVS and DPDK version numbers to current stable versions.

Change-Id: Ib12be3da4f1fc14f1fed990a8abcdf87477a4e1b

openstack/kolla,stackforge/kolla,stackforge/kolla,openstack/kolla,rahulunair/kolla,stackforge/kolla,rahulunair/kolla