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>
tests: Turn off appctl poll_loop logging for long output.

One of the VMware internal autobuilder builds failed due to extraneous
logging in these tests of the form:

   2014-07-28T21:11:07Z|00001|poll_loop|INFO|wakeup due to [POLLIN] on fd 3
   (...) at lib/stream-fd-unix.c:124 (93% CPU usage)

I think this must be because these tests have tons of output and so on a
loaded machine it can take some CPU to pull it down.  We don't want to fail
for that reason, so disable this logging.

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