Support Hadoop-free 1.4 about docker-flink

[clatko]

added hadoopless here - #38

[patricklucas]

There's a bit of trickiness actually introducing the hadoopless images due to how we've done tagging thus far: currently, omitting the "hadoopX" part of the tag implies the latest hadoop variant (currently 2.8), but omitting that part is also how the upstream release archives indicate hadooplessness.

I propose that we introduce the hadoop-free images with Flink 1.5 (due to be released soon), making them the default (i.e. when a hadoop version is not specified), at the same time dropping the 1.3 images.

[iemejia]

+1 @patricklucas It sounds easier to do at the moment of the switch to 1.5. And better not to change the expected labels of 1.4.

[patricklucas]

This is done, though I'm still not sure how to go about making the hadoop images downstream from the hadoop-less ones. I don't think this is a critical issue since it's mostly a disk space optimization.