Create Arbitrary Vault Secrets

[srueg]

Implement the functionality to create arbitrary secrets in Vault for a cluster.

Currently only the cluster token is automatically stored in Vault for a new cluster (from the service account). It should be possible to define arbitrary secrets which are then generated and stored in Vault.
A similar concept as we have to create files in git repos (map of file templates) could be done for Vault secrets. The exact name, length and character set for a secret should be configurable. A secret must be stored under the path <tenant-id>/<cluster-id>/.