agent: Fix agent generate-config to accept namespace

[averche]

Fixing vault agent generate-config to accept -namespace, VAULT_NAMESPACE, and other client-modifying flags.

$ export VAULT_NAMESPACE="admin"
$ vault agent generate-config -type="env-template" -path="path/without/namespace" ...

OR

$ vault agent generate-config -type="env-template" -path="path/without/namespace" -namespace="admin" ...

This addresses VAULT-17295

[averche]

@yhyakuna FYI :)