allow custom labels for jobs

[stefanmaierhofer]

Instead of hardcoded properties, like runtime (win-x64, linux-x64), it should be possible to add arbitrary labels to jobs and nodes.
When scheduling a new job, these labels are used to identitfy appropriate nodes to run the job on.
Custom labels are more flexible than a set of predefined properties, because users have fine-grained control over special node requirements, e.g. a special kind of GPU, or custom OS/library setup, etc.

Examples: win-x64, linux-x64, gpu-nvidia, group1, group2, ...