Implement support for run-untagged option

Sebastian Poxhoferrequested to merge
secustor/gitlab-runner:implement-untagged into master

I have implemented the option to set the executor to run untagged jobs on startup.

The default behavior is unchanged.

To be exact:
If tags are set, the default is false. On the other hand if tags are empty, the default is true.
This can now be overwritten using the untaggedJobs option.

fixes gitlab-org/gitlab-runner#3850 (closed)

Edited by Tomasz Maczukin

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