Allow to switch jwt access token verification and off. Many (most?) IDP
provide JWT encoded access tokens. If ocis is configure to assume jwt
access tokens (access_token_verify_method==jwt) we now properly verify
the tokens signature and a set of standard claims ("exp", "iat" and nbf"
by way of the jwt module's standard verification and "iss" explicitliy).
This change also allows for introduction of other access token verification
mechanism in the future (e.g. through introspection (RFC7662).
The help flag is configured automatically by default already. We don't
need to redo that for every single service.
This also addresses one of the finding of "go race" (#4088)
