Expose maxclients option to command line

[malmond77]

chef-zero is used for spinning up a Chef server for taste-tester (https://github.com/facebook/taste-tester). We've built futher automation around this and have run into practical limits on the number of clients that can reliably retrieve data via https.

Description

chef-zero uses WEBrick to serve https. The default number of request is tied to the default number of threads in WEBrick, which appears to be 100. As this is a subordinate component we need a way to pass in a different value so we can increase the the limit where needed.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] I have run the pre-merge tests locally and they pass.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] All commits have been signed-off for the Developer Certificate of Origin.

There's no new tests to test this, because actually testing the effects of maxclients being set is difficult to verify or falsify - due to timing.

[malmond77]

After further testing, I've found that naively increasing the number for "MaxClients" does not make a significant difference to the amount of concurrent requests that can be served. I am looking at other options for improving performance/scalability. Closing this for now.