vdsm.spec: Adding with_systemd macro

This patch adds the macro with_systemd which can be used
to validate if the system uses Systemd.

Change-Id: Ibcd8567868e16453ed6633f143a6b1fff8e9c454
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12086
Tested-by: oVirt Jenkins CI Server
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>