Separate OpenFlow port numbers from datapath ones.

Separate OpenFlow port numbers from datapath ones.

In a future commit, we will make multiple bridges share a single backing
datapath.  Our simple mapping from datapath to OpenFlow port numbers
won't work, since we'll want the same OpenFlow port numbers on different
bridges.  For example, the OFPP_LOCAL port must be the same on all
bridges, but will have to be a different datapath port on the converged
datapath.

This commit makes it the responsibility of ofproto to assign the
OpenFlow port numbers instead of doing a simple translation from the
datapath ones.

Signed-off-by: Justin Pettit <jpettit@nicira.com>