Runners: publish and use egg information by runner kind

[clebergnu]

Until now, Avocado was limited to run tests on environments such as containers that are packaged in the main egg file "avocado_framework-{version}-py{major}.{minor}.egg".

Now, plugins can describe the assets that contain runners for external and specific test kinds.

At this time, Avocado won't resolve or fulfill dependencies of the assets themselves. For instance, the "robot" plugin requires the "robotframework" Python package. At this time, users have to prepare the environment with the missing asset dependencies.

In the future, other types of runner assets (other than "eggs") might be introduced.