propose solution for: know what's running on other hosts-problem

Signed-off-by: Nico Schottelius <nico@yoda.schottelius.org>

doc/internal/knowledge-about-other-hosts

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+Assume you want to configure stuff one host ("monitor node"),
+depending on the configuration of other hosts ("cluster nodes").
+
+For instance, the monitor host would like to know,
+which hosts are configured with the provider
+"apache" and option --start true.
+
+This requires the monitor node to be able to
+query all other configured nodes. It can't
+ask for all hosts, because cdist does not
+know which hosts are configured or may exist.
+
+Example implementation
+
+If cdist keeps ("caches") the configuration of every
+node it configures, each new node can query the
+cache for existing nodes that acquired the given
+configuration.