podman.service should be an exec service not a notify service

Podman never notifies sytemd that it is ready to recieve connections
so systemd kills it after waiting 1.5 minutes.  Changing to exec
should leave it running until podman exits or the service is stopped.

containers#8751

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

contrib/systemd/system/podman.service

@@ -6,7 +6,7 @@ Documentation=man:podman-system-service(1)
 StartLimitIntervalSec=0
 
 [Service]
-Type=notify
+Type=exec
 KillMode=process
 Environment=LOGGING="--log-level=info"
 ExecStart=/usr/bin/podman $LOGGING system service