add maxParallelism to Disable/Enable-VSTeamAgent

[japj]

PR Summary

Fixes #203 by adding maxParallelism to Disable/Enable-VSTeamAgent

What would be the best way to extend any existing tests for this?

[DarqueWarrior]

Is this a value that the caller should be able to set? We can have it default to 1 but is there a use case where I might want to set it?

[japj]

I don’t know, from the web ui I have only seen a value 1 yet.
It could be that we need to fetch current settings of the agent, patch in the enabled flag and send that back to keep the current setting?

The weird thing is that without this maxparralism gets reset to 0 (which might actually be a server bug?).

[dickvdm]

What are the properties of an agent that are settable by the caller, which are read-only? What does maxParallelism mean anyway ;-)? @bryanmacfarlane, is there documentation on this?

For XAML agents the offline/online was also caller settable, which we loved and used heavily in following scenario:
-Set all agents on a specific host to offline status.
-Do host updates (Operating system, application, configuration, etc.)
-Reboot the host. After reboot the agents on this host would come back to online status again, ready to accept jobs on a freshly updated host :-).

On microsoft/azure-pipelines-agent only enable/disable of agents is caller-settable, offline/online is not afaik. So we have to align host reboot and agent enabling ourselves. As we can't do this in an atomic action, the annoying scenario can occur that scheduled/running builds fail because the host is rebooting.

[DarqueWarrior]

Sorry for delay. I am recovering from surgery. I will get to this ASAP.