Better handling of private registries

[fabric8io]

Better handling of private registries

[jstrachan]

this is similar to #31; if docker.registry isn't specified as a mvn property it'd be nice to default to DOCKER_REGISTRY env var

[rhuss]

Let's reconsider the whole registry addressing stuff:

Currently it is only possible to specify a private registry as part of the image name, but a more flexible naming would be add some value.

My suggestion is:

• If the image name contains a registry, then this registry has the highest precedence.
• Otherwise, if the ImageConfiguration specifies a registry, take this.
• Otherwise, if the global configuration contains a registry, take this for all images.
• Otherwise check the system property docker.registry
• Finally check an environment variable DOCKER_REGISTRY.

If an image is build, and the registry is not specified in the name, it should be tagged with the full name (including the registry found), too.

When it is pushed and the image to push is not 'fully' qualified (i.e. doesn't contain a registry part), but another than the default registry is configured, tag the image locally before pushing to the target registry (if this is required by docker, don't know this by heart)

[rhuss]

DOCKER_REGISTRY is now used as a fallback in 0.11.1 and later, so it should be safe. I'll close that one, please feel free to reopen it if it shouldn't work as expected (and described in the manual).