contabo: fix DNS

we disable the DNS servers in initializeBaseImage. On normal VPS,
unbound seems to start by itself but on contabo it doesn't because
the default unbound config on ubuntu does not work without ip6

src/config.js

@@ -231,7 +231,8 @@ function isManaged() {
 
 function hasIPv6() {
     const IPV6_PROC_FILE = '/proc/net/if_inet6';
-    return fs.existsSync(IPV6_PROC_FILE);
+    // on contabo, /proc/net/if_inet6 is an empty file. so just exists is not enough
+    return fs.existsSync(IPV6_PROC_FILE) && fs.readFileSync(IPV6_PROC_FILE, 'utf8').trim().length !== 0;
 }
 
 // it has to change with the adminLocation so that multiple cloudrons