Update the spec file with the rpm macro to put the unit file
in the system-wide unit file directory based on:
[root@1713bbf19a0b /]# rpmbuild --eval '%{_unitdir}'
/usr/lib/systemd/system
Allow user to create a drop directory to specify options
via environment variables.
Resolves https://github.com/purpleidea/mgmt/issues/12.
