Log by default to journald on systems with systemd (elastic#8942)

Add `-e` flag to command line when running beats from systemd so beats
logs are collected by journald by default in systems with systemd.

Add also some environment variables so command line flags can be
more easily overriden with a drop-in unit or with `systemctl edit`.

Add documentation about the use of Beats with systemd.

docs/setting-up-running.asciidoc

@@ -17,6 +17,7 @@ This section includes additional information on how to set up and run
 * <<directory-layout>>
 * <<keystore>>
 * <<command-line-options>>
+* <<running-with-systemd>>
 * <<shutdown>>
 
 
@@ -28,4 +29,6 @@ include::../../libbeat/docs/keystore.asciidoc[]
 
 include::../../libbeat/docs/command-reference.asciidoc[]
 
+include::../../libbeat/docs/shared-systemd.asciidoc[]
+
 include::../../libbeat/docs/shared-shutdown.asciidoc[]