Add discover feature

[Lawouach]

The current interface of chaostoolkit supports the run of experiments. However, as part of the goal for chaostoolkit, we always wantedt o make it simpler to get into chaos engineering.

The new discover command is aiming to collecting information about a specific target and offer suggestions about potential chaos engineering experiments.

discover has the goal to let look up what an extension is capable of doing, as well as a summary of the platform/application this extension targets (if available) and a list of chaos experiment suggestions.

As not all extensions are Python packages, discover should eventually be able to load a spec file which describes an extension made of process calls or HTTP calls. This may be done in a second iteration of the command.

[russmiles]

In addition there will be a "discover_hook" that can be looked up from a ~/.chaostoolkit/config Yaml file to call up custom toolkit extensions with what has first been discovered from specific extension integrations.