LUCENE-5971: Create addBackcompatIndexes.py script to build and add backcompat test indexes (merged 1627419)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1627438 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5902: rename to addVersion.py

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1627440 13f79535-47bb-0310-9956-ffa450edef68