r/nut{,-monitor}: Enable nut.target

Recent versions of NUT have a *nut.target* unit that collects all of the
NUT-related services.  Enabling any of the services individually does
effectively nothing, as it only adds the service as a `Wants` dependency
for *nut.target*, and that unit already has dependencies for all of
them. Thus, in order for the service to start at boot, *nut.target* has
to be enabled instead.

In situations where only *nut-monitor* should be enabled, enabling
*nut.target* is inappropriate, since that enables *nut-driver* and
*nut-server* as well. It's not clear why upstream made this change (it
was part of a [HUGE pull request][0]), but restoring the desired
behavior is easy enough by clearing the dependencies from *nut.target*.
Services that we want to start automatically can still be enabled
individually, and will start as long as *nut.target* is enabled.

[0]: https://github.com/networkupstools/nut/pull/330

roles/nut-common/files/nut-reset-deps.conf

@@ -0,0 +1,3 @@
+[Unit]
+Wants=
+Requires=