Harvester CLI should better handle namespaced resources in multiple ways

[belgaied2]

Namespaced resources should be handled correctly both when manipulating them directly (get, create, rm) and when referencing them from other resources (necessarily use the format / or similar).

Multiple improvements can be done:

• For the get commands, add a -A, --all-namespaces options to list the resources in all namespaces.
• For all commands, add WARNING when no -n is passed to indicate which namespace is being used
• Force using the format / for resource references.