Add explicit request for groups scope

This patch allows appending request for "groups" scope to the traffic relayed to the identity provider.

Explicit request can be made by:

• command line flag ("groups"),
• environmental variable ("AUTHZ_MEDIATOR_GROUPS"),
• configuration file argument ("groups").

Command line flag argument takes precedence over environmental variable which takes precedence over configuration file argument. Argument should be a valid YAML boolean (yes/true, no/false).

Default behaviour remains unchanged, i.e. no additional scopes are appended to the communication unless explicitly requested.

This patch also makes FakeVerifier structure part of "mediator" package for testing purposes of both internal as well as exported functions.

Signed-off-by: Paweł Wieczorek pawel.wieczorek@collabora.com