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.