Add an configuration option to cap task group workers usage

[ipanova]

Is your feature request related to a problem? Please describe.
Depening on the action, task group can spawn X tasks that will consume all workers and stall whole pulp installation.
Example: ACS task group will create as many tasks as there are repos to refresh.
As a result all workers could be occupied and pulp will not be usable

Describe the solution you'd like
Potentially as an config option that will globally limit how many workers each task group could consume at a time so leave spare resources for other pulp actions - sync, upload, copy, publish, etc.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Here's a brainstorming doc https://hackmd.io/@ggainey/task_throttling_thoughts