Manage worker lease cap as a system variable

[veochen-octopus]

We added an environment variable (OCTOPUS__WorkerLeases__LeaseCap) to help some of the deployment steps spread across more workers. We were hoping that this would help us determine what would be a good default value so we can eventually get rid of the variable. It became obvious that there is no such a value for all customers. There are valid cases for no distribution, full distribution (round robin) and anywhere in between. So the variable is here to stay. And an env var is too clumsy for that. We should add a system variable for it, which has the added benefit of being self-serve and applying to individual projects rather than the whole instance.

Relates to #7569

[octoreleasebot]

Release Note: Add variable Octopus.Deployment.WorkerLeaseCap to manage worker lease cap

[veochen-octopus]

The new system variable is called Octopus.Deployment.WorkerLeaseCap. Docs to follow.

[Octobob]

🎉 The fix for this issue has been released in:

Release stream	Release
2023.1	2023.1.11423
2023.2	2023.2.12643
2023.3	2023.3.5093
2023.4+	all releases