Docker username required when pushing to Hub

[nikhilm]

Using lambda with the default configuration of Docker Hub + IronWorker requires that the image name have a username/ in it. Currently the blog post and the docs do not make this explicit enough.

1. Update docs

I do not agree with enforcing that all images created via ironcli have a username. This is because users may be using private registries or not deploying to IW. Rather I recommend the following:

1. The aws-import command already accepts a -image flag to specify an alternate name. This will be made clear in the docs.
2. If publish-function detects an "invalid" image name, print out a nice message explaining what is wrong, and also print the exact docker command the user can execute to fix the problem. This is because publish-function is the only thing affected by this requirement.
3. Optional: Add a flag to aws-import and create-function that picks up the login credentials for Hub from docker configuration and auto prefixes images.

[nikhilm]

Also added note here d2288f0

@supereli