Basic support for using kubernetes passwords using the OpenShift client

[dwimsey]

This modification simply calls oc whami -t to get the kubernetes API token (which also is used for the internal docker registry authentication) and uses that for the users password when the associated server entry in the maven settings has a password of 'oc'.

Documentation and multiple context support to follow

[mattnworb]

Thanks for the PR. Do you happen to know if openshift support for a registry push can also be achieved by configuring docker, either by editing config.json or a credentials helper?

The latter is the path we have been taking recently to add support for other registry providers, to avoid needing to add code for each new provider/API/tool into the plugin directly.