ovs-appctl: Add logging options.

ovs-appctl: Add logging options.

Normally I would also add documentation for the logging options to the
ovs-appctl manpage, but I am concerned that in this case it would actually
make the manpage confusing, because one of the main purposes of ovs-appctl
is to modify the log levels of *other* programs, and these options only
modify the log level of ovs-appctl itself, which is rarely useful.

The following commit will start using these logging options in a test.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ansis Atteka <aatteka@nicira.com>