Refactor repo structure

[pomadchin]

Resolves first part of the https://github.com/geotrellis/geodocker-cluster/issues/5 issue.

• [x] refactor repos (and remove 2 and 3 nodes zookeepers images)
• [x] change download links for serf and spark (issue #11)
• [x] scala 2.11 support (spark 2.11 distribution, issue #7)
• [x] move cfgs to folders specific only to major releases (1.6.4 => 1.6; 1.7.0 => 1.7) to provide Accumulo 1.6.x and Accumulo 1.7.x support
• [x] publish to dockerhub
• [x] test