service: Don't shutdown the VM when agent terminates

The semantic around the agent is that it should be a passive
component, hence it should not implicitly shut down the VM.

Instead, we expect the kata-runtime to be responsible for this,
using the appropriate VM interface to stop it.

Fixes kata-containers#449

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

kata-agent.service.in

@@ -10,5 +10,4 @@ StandardOutput=tty
 Type=simple
 ExecStart=@bindir@/@kata-agent@
 LimitNOFILE=infinity
-ExecStop=/bin/sync ; /usr/bin/systemctl --force poweroff
-FailureAction=poweroff
+ExecStop=/bin/sync