Add support for images context semantic validation

This patch adds support for images context semantic validation, it's one
of the series patches for the bp:
https://blueprints.launchpad.net/rally/+spec/benchmark-context-semantic-validation.
It just lists images to check whether the image service is available.

Partially Implements: blueprint benchmark-context-semantic-validation

Change-Id: Iecd54dc96110e5ce592478601f3c601e4275768d
Merge "Add support for images context semantic validation"