Define allowed params per service instead of in ServiceParams

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We currently have a big list of allowed params for services but some of these params are specific to certain services only, for instance :teamcity_url or :bamboo_url.

#service_params should return only the params a service is expecting. Each service could define specific params it's expecting (the list of shared params can stay in ServiceParams), and then ServiceParams#service_params would ask the service for its allowed params (e.g. @service.allowed_params).