Hardcoded "administrator" account when joining domain

During troubleshooting why joining my domain fails, I found in `init.sh` that there is a hardcoded account name specified: `-U"${URDOMAIN}\administrator"`. Although this might be correct in many cases, it causes trouble when the domain's admin account is named differently.

So my demand is to introduce an environment variable which allows for declaring the username to use for joining the domain.