Support variables in PipelineSchedule

[dawi]

It would be nice to support PipelineSchedule Variables.

https://docs.gitlab.com/ce/api/pipeline_schedules.html

Keep up the great work. This is such a useful project!!! :)

[gmessner]

@dawi
Support has been added for pipeline schedule variables and is available in release 4.9.20. Please verify and close when you have a chance.

[gmessner]

@dawi
Since this feature is available in the latest release I am closing this issue, if you find any problems with the implementation, please feel free to re-open this issue.