LU-9526 spec: Improve systemd compat in spec file

LU-9526 spec: Improve systemd compat in spec file

Few minor edits that make the systemd compatibility
in the lustre.spec file cleaner.

Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Change-Id: I02076c7806f2c7cb318a505943dca0ca5a030093
Reviewed-on: https://review.whamcloud.com/27215
Reviewed-by: Christopher J. Morrone <morrone2@llnl.gov>
Tested-by: Jenkins
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>