Install erlang from Ubuntu repos

With Ubuntu Trusty we can install Erlang R16B3 from the Ubuntu repositories.
Expose database volume
Run as user ejabberd
Cleanup when done
Add more environment variables

* LOGLEVEL: Set the ejabberd loglevel
* ERLANG_NODE: Set noe to hostname
* ERLANG_COOKIE: Set an erlang cookie
Set local var
Chang readme to use this image as base image
Remove proposed repository
Use ejabberd users erlang cookie file