MT#6389 Add ability to stip DNS check via SKIP_DNS_CHECK_TEST

We have no DNS records for freshly installed VMs/Vagrant/Amazon images.
So, we have to skip the test there. It is enabled by default for
other test cases as it is useful itself.

We can conside to configure DNS for freshly installed VMs one day.

pro/01_hostname-and-interfaces.t

@@ -64,6 +64,7 @@ sub main {
 	}
 
 	SKIP: {
+
 		skip 'fqdn not found', 2
 			if not defined $hostname_fqdn_ip;
 
@@ -88,6 +89,9 @@ sub main {
 				if $ENV{TEST_VERBOSE};
 		}
 
+		skip 'DNS resolve check as requested via env SKIP_DNS_CHECK_TEST=true', 1
+			if $ENV{SKIP_DNS_CHECK_TEST};
+
 		# check if every interface has a hostname set (different from the ip)
 		eq_or_diff(
 			[ map {