LU-11575 build: install systemd stuff only for debian with systemd enabled

Add a precheck for systemd, to avoid to try to package systemd
stuff intolustre-client/server-utils deb when building on debian
series without systemd supported.

Test-Parameters: clientdistro=ubuntu1604 trivial

Change-Id: If58b64acc035e621594ab420a8b900b18a34a211
Signed-off-by: Gu Zheng <gzheng@ddn.com>
Reviewed-on: https://review.whamcloud.com/33492
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>