ovn-controller: Factor patch port management into new "patch" module.

ovn-controller: Factor patch port management into new "patch" module.

Upcoming patches will introduce new extensive use of patch ports and it
seems reasonable to put it into its own file.

This is mostly code motion.  Code changes are limited to those necessary
to make the separated code compile, except for renaming
init_bridge_mappings() to patch_run().

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>