Fix incorrect path for geard-image service file.

The geard-image service file refers to /etc/systemd/systemd instead
of /etc/systemd/system. This causes attempts to start or restart a
service over the HTTP interface to fail:
alter_container_state: Unable to persist whether the unit is started on boot: symlink /var/lib/containers/units/te/ctr-test-1.service /etc/systemd/system/container-active.target.wants/ctr-test-1.service: no such file or directory