Multi-namespace support for Vault

By setting the namespace in the vault section, we lock ourselves into a single namespace.

Vault clients support namespace as a path prefix
Which would be the right way to access secrets in multiple namespaces from a single consul template config.

I use the latest version.

The issue names no files, tests, or entry points. Start by tracing how the vault section configures the Vault client and how namespace path prefixes are handled; done means one Consul Template configuration can access secrets from multiple Vault namespaces.

Written by the indexing model from the issue text.