Seperate registry handling for pull and push

[rhuss]

Currently we can provide only a single registry via configuration or system properties. However there are use case for which a pull and and a push can occur as a result of a single mvn call. E.g. mvn docker:build docker:push could potentially autopull base images for a registry A and push the resulting images to registry B.

This also implies that there can be two different set of authentication credentials are required, or maybe if no credentials at all.

In practice this is not so much an issue because authentication configuration comes from settings.xml or ~/.docker/config.json where multiple authentication specs can be stored (and looked up by registry). Also for pulling images which don't require an authentication but the (correct) authentication is send nevertheless doesn't harm (although not needed).

But for auth props given on the command line there is currently only one set which can be used (docker.username and docker.password) and its not clear whether it should be used for pull or push (albeit push is more likely of course).

Suggestion is:

• Introduce a property docker.noAuthentication with values pull, push or all
• Introduce a properties docker.pull.username and docker.pull.password (same for push) which overrides docker.username and docker.password if given. The latter are still used as default.

This issue was first mentioned in #147 . This is the followup issue.

Any opinions on this ?