SLING-3760 : improved installBundles with checks as to whether the bundle already exists on the server. If it exists in the very same version (excluding SNAPSHOT), then the bundle is left untouched. If it exists but in a different version or a SNAPSHOT version, the bundle is first uninstalled, then installed. If it doesn't exist yet, then it is just installed (as was done before)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1610606 13f79535-47bb-0310-9956-ffa450edef68