Simplify validity check for loglevel expression.
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 <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

mschoch/ns_server,ceejatec/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,membase/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server