Add experiment requirements and agent capabilities

[pietroalbini]

Now that we have Windows support in Crater we need a way for Craterbot to schedule Windows runs on Windows agents and Linux runs on Linux ones. A way to do that is to add "capabilities" to agents and the ability for experiments to require some capabilities to be present in the scheduled agents.

A way to implement this is to add a capabilities argument to the /next-experiment Agent API call and a requires field to experiments. When the Agent API needs to choose an experiment to run on the agent it will only return one the agent can properly fulfill.

This is part of #149.