Adds support for custom image formats in scenarios

Scenario Tempest tests don't work on Hyper-V due to the missing
support for VHD/VHDX image formats in the Tempest scenario comfiguration.

The commits adds options to specify custom image disk and container
formats, mantaining 'QCoW2' and 'bare' as the default options for
backwards compatibility.

Closes-bug: #1357691
Co-Authored-By: Ionut Balutoiu <ibalutoiu@cloudbasesolutions.com>
DocImpact

Change-Id: I41c09d5a682cdb068b97b4bd1be3403ee7973c7c
Merge "Adds support for custom image formats in scenarios"