Document the differences between the various variants

[prakashsurya]

We should do a better job documenting the various variants that we produce, and also how each variant should be used. E.g. internal-qa should be used for all QA automation rather than internal-dev, since internal-qa more closely matches a customer's system.

We should also help map the appliance-build variant names to dcenter image names, since some folks will interact with dcenter images rather than directly with appliance-build variants.