SLING-3035 - Eclipse build - target feature cleanup

- merge the two duplicate indigo update sites into a single one
- remove the unneeded 'Feature' location from the target definition
- use a stable update site for m2e as the main one is refreshed on each
release
- move to a slightly older m2e version to allow installation in older
versions

git-svn-id: https://svn.apache.org/repos/asf/sling/branches/tooling-ide-vlt@1522835 13f79535-47bb-0310-9956-ffa450edef68