debian: Automatically start openvswitch before first invocation of ovs-vsctl.

In the 'interfaces' file, if an admin adds the openvswitch interface
in 'auto', ifupdown will try to create OVS interfaces even before
openvswitch has started. In a case like that, assume that the admin
knows what he is doing and try to start openvswitch.

The negatives I see are
1. /usr is NFS mounted, in which case expect the admin
to mount it through initramfs.
2. syslog through network may not be accessible.

This is similar to what rhel does with commit 602453000e28ec10
(rhel: Automatically start openvswitch service before bringing an ovs
interface up or down)

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
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>