Support for simple user provided maven server

[jam01]

Straight forward, it supports the use case where we don't have a dedicated mirror or proxy and instead have credentials to the maven server directly. I understand we could use the custom settings.xml feature but figured this might be acceptable.

[rhuss]

@jam01 could you please add a paragraph to https://github.com/fabric8io-images/s2i/blob/master/java/templates/README.md#build-time explaining the new variables ? (sorry that I've merged to early)

[rhuss]

And if possible recreate the fish-pepper related files with

docker run -it --rm -v `pwd`:/fp fabric8/fish-pepper

(or with a locally installed fish-pepper)

Thanks !