Add support for VAULT_NAMESPACE environment variable for automatic namespace selection

[dereckson]

hvac library uses environment to configure Vault connection properties.

Currently, hvac.Client() will detect Vault URL from the VAULT_ADDR environment variable, but doesn't detect namespace from the VAULT_NAMESPACE variable.

In an automation workflow, if we want to customize namespace according environment, that creates this code:

vault_client = hvac.Client(namespace=os.environ["VAULT_NAMESPACE"])

At worst, if namespace is optional:

try:
    vault_client = hvac.Client(namespace=os.environ["VAULT_NAMESPACE"])
except KeyError:
    vault_client = hvac.Client()

[dereckson]

Is that something acceptable for the software?

If so, I could prepare a pull request for this feature.