Switch to systemd by default

Closes: https://github.com/CentOS/centos-bootc/issues/449

centos-bootc-config.json

@@ -4,5 +4,6 @@
         "redhat.compose-id": "CentOS-Stream-9-20240304.d.0",
         "redhat.id": "centos",
         "redhat.version-id": "9"
-    }
+    },
+    "StopSignal": "SIGRTMIN+3"
 }

fedora-bootc-config.json

@@ -4,5 +4,6 @@
         "redhat.compose-id": "Fedora-ELN-20240326.3",
         "redhat.id": "fedora",
         "redhat.version-id": "ELN"
-    }
+    },
+    "StopSignal": "SIGRTMIN+3"
 }

tier-0/manifest.yaml

@@ -19,7 +19,7 @@ etc-group-members:
 
 # Default to `bash` in our container, the same as other containers we ship.
 container-cmd:
-  - /usr/bin/bash
+  - /sbin/init
 
 # Note that the default for c9s+ is sqlite; we can't rely on rpm being
 # in the target (it isn't in tier-0!) so turn this to host here.  This