selftest: correctly use RESOLV_CONF from the DC environment or "no_resolv.conf"

selftest: correctly use RESOLV_CONF from the DC environment or "no_resolv.conf"

It's important to have the correct $resolv_conf variable within
provision(), because it also sets "RESOLV_WRAPPER_CONF" if needed,
instead of just setting "RESOLV_CONF".

There's also no point in creating an resolv.conf with the ip addresses
for other roles than "active directory domain controller".

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>