MB-8883 Don't crash in testEmail.

Since recently menelaus_alert:parse_settings_alerts_param stopped
accepting unknown parameters. And "subject" and "body" are exactly
unknown to it. So they need to be removed from the args passed to that
function.

Change-Id: I0900e705ea95c8bcc81a1351845ae13899135ea7
Reviewed-on: http://review.couchbase.org/28411
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>