ovs-ctl: Allow selective start for db and switch

ovs-ctl: Allow selective start for db and switch

Currently, ``ovs-ctl start'' will attempt to start both the DB and
vswitchd. This is quite convenient when the database already has all of
the configuration values required, and when using a single services file
for systemd integration. The same goes for the ``ovs-ctl stop'' command.

However, there are some cases which are not easily covered. The case
where we want to set values in the database prior to starting the
forwarding path, as well as the case of supporting multiple service
files, one per daemon (which is how systemd expects services to look).

Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>