debian: Don't depened on $RUNLEVEL at startup to create bridges.

Commit b2a0daa5bd (debian: Don't recreate bridges during manual restart.)
added a check on $RUNLEVEL to only create bridges and ports when the
system starts up. This fix does not work with systemd.

This commit uses a different approach to solve the same problem.

Reported-at: https://bugs.debian.org/686518
Reported-by: Philipp S. Schmidt <phils@in-panik.de>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Tested-by: Philipp S. Schmidt <phils@in-panik.de>