Currently the vault integration does not support Vault EE namespaces.
Only the vault server URL, role name and auth path can be configured
with the variables. It is possible to just prefix the engine's path
with the namespace, but it is not possible to specify the namespace
for the authentication - it always uses the root namespace.

This change makes the namespace configurable wiht the VAULT_NAMESPACE
variable (similar to how `VAULT_SERVER_URL`, `VAULT_AUTH_ROLE` and
`VAULT_AUTH_PATH are being used already). It will fall-back to an
empty string in case the variable is not defined by the user.

Fallback to an empty string instead of 'root' when no vault namespace specified by the user.

Changelog: added