Prevent dollar signs and single quotes in variables from being removed by shell

Change-Id: I88782bd60755f96816bf9d06cd57298cb46f8e40
Add support for alternate images

Tempest runs can utilise two distinct images in glance.
This patch adds the option for a user to specify
a second name to be used with the current image_source, or
a second name in addition to a second image_source_alt.

Change-Id: I8b7d3b77f07d39ee5944e00b47e7ab1af67f7be7
Merge "Prevent dollar signs and single quotes in variables from being removed by shell"