ssh-host-keys: Do not specify a namespace

We don't want to hard-code a namespace for the `ssh-known-hosts`
ConfigMap because that makes it less useful for other projects besides
Jenkins.  Instead, we omit the namespace specification and allow
consumers to specify their own.

The _jenkins_ project doesn't have a default namespace, since it
specifies resources in the `jenkins` and `jenkins-jobs` namespaces, we
need to create a sub-project to set the namespace for the
`ssh-known-hosts` ConfigMap.

ssh-host-keys/kustomization.yaml

@@ -3,7 +3,6 @@ kind: Kustomization
 
 configMapGenerator:
 - name: ssh-known-hosts
-  namespace: jenkins-jobs
   files:
   - ssh_known_hosts
   options: