Improve plugins and apply loading for runners and contexts plugins

Provide plugins loading for runners and contexts, improve samples.
Plugins now can be loaded from ~/.rally/plugins and /opt/rally/plugins
and any it's subdirectories.

Closes-Bug: #1320942

Change-Id: I213086e7eae17e297c398c301f5ed8d2331d951e
Merge "Improve plugins and apply loading for runners and contexts plugins"