Fixes case sensitivity issue in legacy lucene index paths

The path under which legacy indexes are stored was changed
 to have an uppercase first letter for the entity type that
 is stored by the index (so, Node instead of node, for example).
 This causes migration from previous versions to fail to properly
 pick up copied indexes, including auto indexes, of course only
 on case sensitive filesystems.
This patch reintroduces the old lowercase naming scheme so that
 paths are built and found as before.
Merge pull request #3349 from digitalstain/2.2-fix-index-path-case

Fixes case sensitivity issue in legacy lucene index paths