After noticed non-root user can list devstack services in plugin's
triggered jobs, fixed past review comment to use preferred `systemctl`
instead of configurable directory existence check.
Added common local service check function to reduce repeated code.
Small code improvements along the way, like sparing second shell
call when podified setup used.
Change-Id: Idbb98765ef2020d674ed00f3b43193d0bed935d2
6cdf917a6d