Add support for the VAULT_NAMESPACE environment variable

If the Client class is instantiated without the namespace argument, use
the content of the VAULT_NAMESPACE environment variable instead,
mimicking the behaviour of the official vault CLI tool.

docs/overview.rst

@@ -30,16 +30,6 @@ Using TLS with client-side certificate authentication:
     True
 
 
-Using `Vault Enterprise namespace <https://www.vaultproject.io/docs/enterprise/namespaces/index.html>`_\ :
-
-.. doctest:: init
-
-    >>> client = hvac.Client(
-    ...     url='https://localhost:8200',
-    ...     namespace=os.getenv('VAULT_NAMESPACE'),
-    ... )
-
-
 Using plaintext / HTTP (not recommended for anything other than development work):
 
 .. doctest:: init

hvac/v1/__init__.py

@@ -72,6 +72,7 @@ def __init__(
 
         token = token if token is not None else utils.get_token_from_env()
         url = url if url else os.getenv("VAULT_ADDR", DEFAULT_URL)
+        namespace = namespace if namespace else os.getenv("VAULT_NAMESPACE")
 
         if cert is not None and VAULT_CLIENT_CERT:
             cert = "\n".join(