dns_aws: Fix when _acme-challenge is a hostedzone

The function '_get_root' tries to retrieve the
    hostedzone iterating the domains, eg:
      1. srv.prod.example.com
      2. prod.example.com
      3. example.com
    This doesn't work if '_acme-challenge' is in it's
    own hostedzone for security reasons.
    Starting that iteration with '_acme-challenge.srv.prod.example.com'
    fixes this issue.

dnsapi/dns_aws.sh

@@ -151,8 +151,8 @@ dns_aws_rm() {
 ####################  Private functions below ##################################
 
 _get_root() {
-  domain=$1
-  i=2
+  domain=_acme-challenge.$1
+  i=1
   p=1
 
   if aws_rest GET "2013-04-01/hostedzone"; then