More sensible default variables

I have implemented changes proposed in #7, is it okay? The one particular thing I am not sure about is handling of the root db password – currently the tasks related to password are skipped when the password is empty, but I was also considering to set the `mysql_root_db_pass` variable as mandatory (and then it would be better omitted from defaults completely).

However this change can distrupt existing usages of this role, so I recommend to tag your current version before merging, so Galaxy users can easily revert to the old version.