Add test that verifies that maxpasswd does not output garbage.

At least on 1.4.1, masspasswd outputs additinal information in addition to the key:

1. maxpasswd /var/lib/maxscale/data/ mypassword
   6E37E9B0CA3BF6020A3DBCD0DE113546
   2016-03-31 17:03:23 notice : Using encrypted passwords. Encryption key: '/var/lib/maxscale/data/.secrets'.

And everything goes to stdout. That makes it difficult to use maxpasswd in scripts.

Add test-case that verifies that maxpasswd outputs the key and the key alone.