ofproto: Add initialization function.

A future commit will make all bridges of a particular dpif share a
single backing datapath.  In order to handle restart, the datapath will
need to have some idea of what the initial state looks like.  Otherwise,
it won't know which ports belong to which bridges and orphaned ports may
never be cleaned up.

This commit introduces an initialization method to ofproto, which takes
as an argument a high-level description of the bridges and ports.  An
ofproto provider can then use this information to initialize its state.

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