Simplify validity check for loglevel expression.

Reject only atoms that are not one of the known loglevels.

Change-Id: Iba5c6b20b2a98864e5aad1512568d7b5839aba54
Reviewed-on: http://review.couchbase.org/37520
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>