add missing depends_on namespace-defs

This MR adds a depends_on: - namespace-defs on a few units that were not having it, although they were deploying things in a namespace defined in namespace-defs.

This is necessary because the pod-security.kubernetes.io/* labels set on the namespace are important to allow some units to start, and for at least some namespaces, these are defined in the namespace-defs unit.

Some of the units touched by this MR already had an "indirect depends_on" to namespace-defs (they had a declared depends_on on a unit itself depending on depends_on) - for those units, this MR is merely a cleanup (it's better to always declare all dependencies explicitly).

For the following units, this MR is actually a fix that will help avoid race conditions during startup:

• monitoring-crd (will contribute to closing #886 (closed))
• vault-operator and vault
• sriov
• longhorn