Update ejabberdctl.rb to properly handle a user already existing.

In my puppet code I am attempting to add two users.

   ejabberd_user { $::fqdn:
     host        => $::fqdn,
     password    => 'examplepass',
   }
   ejabberd_user { "admin":
     host        => $::fqdn,
     password    => 'examplepass'
   }
The users are added properly but on the next puppet run the following error happens:

Error: Execution of '/usr/sbin/ejabberdctl register admin fakehost.net examplepass' returned 1: User admin@fakehost.net already registered at node ejabberd@fakehost.net

I believe my code fixes this error.