integrate revamped authentication support from docker-client

[mattnworb]

This largely mirrors changes done in dockerfile-maven to integrate the
new authentication support from docker-client.

The plugin now loads credentials from three places, in order:

1. Any credentials stored in ~/.dockercfg or ~/.docker/config.json

2. (Optional) Google Cloud credentials, either loaded through the
   DOCKER_GOOGLE_CREDENTIALS environment variable or Google's automatic
   resolution process for credentials

3. Last precedence is given to any explicitly configured credentials in
   the plugin's configuration.

The useConfigFile parameter is no longer used as authentication info
from the docker configuration file is always activated.

One of the big motivations of the revamped authentication support in
docker-client was to address long-standing issues in this plugin in
regards to being able to authenticate against multiple registries, as
the previous implementation assumed that the same credentials would be
used for any possible registry.

Closes #326, #284, #252, and probably more.

For more reference see:

• integration with pushing/pulling images from Google Cloud Registry docker-client#740
• use google-auth-library-oauth2-http for fetching access tokens docker-client#762
• default to pulling auth info from docker config file docker-client#775
• document RegistryAuthSupplier in the user manual docker-client#781
• WIP: adds support for pushing to GCR via mvn deploy. Has failing in… dockerfile-maven#13
• support for loading Google credentials from environment file dockerfile-maven#17

[mattnworb]

@davidxia @caipre @mavenraven

[davidxia]

if you're confident various cases work end-to-end