Add some helpers for env identification

[desaintmartin]

• define environment types: an Enum or sth else. (to unify and to avoid using strings "staging"; "prod" ...
• Return en env type of a namespace (from labels probably)
• Tell if a namespace is prod/staging (to avoid having sometimes if "prod" in namespace: sometimes if "-prod-" in namespace:) => If we make a change we make it in one place.
• Given a resource name, return the env type it belongs to (maybe add the env as a label)
• environmentkind in helmfile.yaml (used to construct the appropriate priorityclass, dev_mail ...)
• environmentkind in cluster-manager (used by mayo, and other tools)
• some teams (cayzn) define their own EnvironmentKind Enum, how/what should we do to keep being sync?)