Use an IP address for Google instance metadata (#521)

* Use an IP address for Google instance metadata

Makes Spilo start even if external DNS queries from
within the container are not working.

Do not continue with the startup when /etc/service is missing.

Per https://github.com/zalando/spilo/issues/511



* Follow Python guidelines on on-line comments
Co-authored-by: Alexander Kukushkin <cyberdemn@gmail.com>

* Extra space to the shell comment for readability
Co-authored-by: Alexander Kukushkin <cyberdemn@gmail.com>

postgres-appliance/launch.sh

@@ -53,6 +53,8 @@ sv_stop() {
     sv -w 86400 stop /etc/service/*
 }
 
+[ ! -d /etc/service ] && exit 1  # /etc/service has not been created due to an error, the container is no-op
+
 trap sv_stop TERM QUIT INT
 
 /usr/bin/runsvdir -P /etc/service &

postgres-appliance/scripts/configure_spilo.py

@@ -376,7 +376,7 @@ def get_instance_metadata(provider):
     headers = {}
     if provider == PROVIDER_GOOGLE:
         headers['Metadata-Flavor'] = 'Google'
-        url = 'http://metadata.google.internal/computeMetadata/v1/instance'
+        url = 'http://169.254.169.254/computeMetadata/v1/instance'  # metadata.google.internal
         mapping = {'zone': 'zone'}
         if not USE_KUBERNETES:
             mapping.update({'id': 'id'})