Support safe introduction of secrets

[eigengrau]

It would be convenient if there were a way to safely introduce secrets (e-mail password, Slack token), which must currently be stored in the plain within Consul (while access can be mediated via ACL, secrets will still be stored in the plain on the underlying Consul servers).

In case you do not like to introduce any configuration over a non-Consul channel (static configuration file, environment variable), perhaps it would make sense supporting Consul-Template expressions within configuration values stored in Consul. This would allow secret introduction via expressions like {{ with secret … }}.