Introduce userExistsInConf in AuthService

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Introduce userExistsInConfOrAuth in AuthService

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Introduce listUsersFromConf in AuthService

Please reread/test this one carefully.

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Switch from AuthService::listUsers to AuthService::listUsersFromConf
and from authCountUsers to getUsersCountFromConf

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Replace AuthService::userExists with AuthService::userExistsInConf
and AuthService::userExistsInConfOrAuth

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>