ocf-shellfuncs: systemd_drop_in only if needed

Avoid dbus overload upon many simultaneous "daemon-reload" invocations
(when a resource agent using systemd_drop_in() is called multiple times
as part of parallel resource operations in Pacemaker) by skipping the
file creation and reload if the expected data already exists.

Whilst at it, align the indentation of the heredoc with the other parts
of the function.

Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>