Draft: Support Vault EE namespaces

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.

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). API operations performed under a namespace are done by providing the namespace name in X-Vault-Namespace` header.