Packaging improvements & bugfixes

* RPM: Use the ES_USER variable to set the user (same name as in the debian package
  now), while retaining backwards compatibility to existing /etc/sysconfig/elasticsearch
* RPM: Bugfix: Remove the user when uninstalling the package
* RPM: Set an existing homedir when adding the user (allows one to run cronjobs as this user)
* DEB & RPM: Unify Required-Start/Required-Stop fields in initscripts