Add multiple service definition support

This change-set adds another key to the configuration decoding called
`services`, which is expected to be a list of service definitions. It
follows the established convention of only allowing one of the keys:
`service`, `check`, `services`. For every entry in the list it calls
the corresponding decode method and appends it to the Servics of the
resulting Config.

While a similar result could be achieved with changing the Services
member of the Config struct to have named mapstruct tag it lacks the
proper tinme conversions provided by DecodeServiceDefinition.