ovs-appctl: register unixctl commands in the beginning

ovs-appctl: register unixctl commands in the beginning

Some commands of ovs-appctl were lazily registered when first
bridg or bfd was added. Before that, calling these commands raised a
error("xxx is not a valid command"). The problem commangs included
"bfd/...", "upcall/...","dpif/...","fdb/..." and so on.

Fix this by moving the register into the "bridge_init" and
"bridge_init_ofproto". All commands are registered at the moment
ovs-vswitchd starts.

Signed-off-by: Huanle Han <hanxueluo@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>