Publish immutable artifact tags + canary version to Docker registries

[sgoings]

We want to have the ability to have deis/charts reference a specific immutable image which will enable us better traceability on issues that come into various components.

What we've done with efforts in other repositories is publish a version:

• git-[short sha] that will never be overwritten
• canary that will move to reference the latest version available of a component

Examples of the easiest way to roll this change out can be seen in deis/workflow#487, deis/builder#246, and deis/workflow-manager#11

[jchauncey]

This repo already publishes the git-sha tag for each Travis run. How are we going to handle the canary tag?