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 time conversions provided by DecodeServiceDefinition.
Merge pull request #431 from soundcloud/xla-multiple-service-definitions

Add multiple service definition support