jenkins: Add ssh_known_hosts ConfigMap

When cloning/fetching a Git repository in a Jenkins pipeline, the Git
Client plugin uses the configured *Host Key Verification Strategy* to
verify the SSH host key of the remote Git server.  Unfortunately, there
does not seem to be any way to use the configured strategy from the
`git` command line in a Pipeline job, so e.g. `git push` does not
respect it.  This causes jobs to fail to push changes to the remote if
the container they're using does not already have the SSH host key for
the remote in its known hosts database.

This commit adds a ConfigMap to the *jenkins-jobs* namespace that can be
mounted in containers to populate the SSH host key database.

jenkins/kustomization.yaml

@@ -10,3 +10,11 @@ secretGenerator:
   type: kubernetes.io/dockerconfigjson
   files:
   - .dockerconfigjson
+
+configMapGenerator:
+- name: ssh-known-hosts
+  namespace: jenkins-jobs
+  files:
+  - ssh_known_hosts
+  options:
+    disableNameSuffixHash: true