Refer to database manpages in *ctl manpages

Refer to database manpages in *ctl manpages

The ovn-nbctl, ovn-sbctl, and ovs-vsctl manpages are inconsistent in
their "Database Commands" section when it comes to referring to what
database tables exist. This commit amends this by making each *ctl
manpage reference the corresponding database manpage instead.

To aid in having a more handy list, the --help text of ovn-nbctl,
ovn-sbctl, and ovs-vsctl have been modified to list the available
tables. This is also referenced in the manpages for those applications.

Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>