leap test: add test for mx node to check couch connection

leap test should discover a not authorized couchdb accesss (which we currently have on leech, the production mx node (see #4926).

currently, only networking + stunnel are checked for mx nodes:

 = [leech] PASS: Network > Can connect to internet
 = [leech] PASS: Network > Stunnel is running
 = [leech] 2 tests, 5 assertions, 2 passes, 0 failures, 0 errors, 0 skips

(from redmine: created on 2014-01-09, closed on 2015-02-03, relates #4926)