Allow Setting Vault Namespace through Annotation

[malnick]

In order to set a namespace today, you need to do so through the secret annotation setting:

{{- with secret "`ns1/ns2/secret/foo" -}}
        {{.Data.password}}
{{- end }}

Consider adding an annotation that does this explicitly for the agent for overriding VAULT_NAMESPACE such as:

vault.hashicorp.com/namespace: "ns1"

Then you would only need to write {{- with secret "secret/foo" -}} and ns1 would be added to the path.

[malnick]

Duplicate of #20