Support use of VAULT_NAMESPACE env var (#929)

As per https://learn.hashicorp.com/tutorials/vault/namespaces, setting VAULT_NAMESPACE env var is a completely supported mechanism to make all vault command use said namespace, so hashi_vault lookup function should do the same.

Co-authored-by: Holt Wilkins <hwilkins@palantir.com>

changelogs/fragments/929-vault-namespace-support.yml

@@ -0,0 +1,3 @@
+---
+minor_changes:
+  - hashi_vault - support ``VAULT_NAMESPACE`` environment variable for namespaced lookups against Vault Enterprise (in addition to the ``namespace=`` flag supported today) (https://github.com/ansible-collections/community.general/pull/929).

plugins/lookup/hashi_vault.py

@@ -117,6 +117,9 @@ DOCUMENTATION = """
       default: True
     namespace:
       description: Namespace where secrets reside. Requires HVAC 0.7.0+ and Vault 0.11+.
+      env:
+        - name: VAULT_NAMESPACE
+          version_added: 1.2.0
     aws_profile:
         description: The AWS profile
         type: str