[packaging][centos] Improve service (systemd + init script) (#3477)

* Make agent service depend on fs and net services

This should hopefully ensure that the agent doesn't start too early in
the system startup process.

* Move systemd service files up one directory

They're common to debian/centos/suse now.

packaging/centos/datadog-agent.init

@@ -16,8 +16,8 @@
 # Provides: datadog-agent
 # Short-Description: Start and start datadog-agent
 # Description: datadog-agent is the monitoring agent component for Datadog
-# Required-Start:
-# Required-Stop:
+# Required-Start: $remote_fs $network
+# Required-Stop: $remote_fs $network
 # Default-Start: 2 3 4 5
 # Default-Stop: 0 1 6
 ### END INIT INFO