parsemail: Fix default value of verbosity argument

parsemail: Fix default value of verbosity argument

With the current parsemail.py, the default value for the verbosity
argument does not exist in the VERBOSITY_LEVELS dict, so we get:

  Traceback (most recent call last):
    File "./patchwork/bin/parsemail.py", line 569, in <module>
      sys.exit(main(sys.argv))
    File "./patchwork/bin/parsemail.py", line 555, in main
      logging.basicConfig(level=VERBOSITY_LEVELS[args['verbosity']])
  KeyError: 20

This change uses an actual key instead.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Stephen Finucane <stephen.finucane@intel.com>