systemd: merge controller/worker config examples (#3342)

Our systemd setup included both a controller and worker
configuration example, and a %i templated config file meant
to be used to determine which to use. For ease of use, merge
the two config files into a predictably named "boundary.hcl".

.release/linux/package/usr/lib/systemd/system/boundary.service

@@ -10,7 +10,7 @@ User=boundary
 Group=boundary
 ProtectSystem=full
 ProtectHome=read-only
-ExecStart=/usr/bin/boundary server -config=/etc/boundary.d/%i.hcl
+ExecStart=/usr/bin/boundary server -config=/etc/boundary.d/boundary.hcl
 ExecReload=/bin/kill --signal HUP $MAINPID
 KillMode=process
 KillSignal=SIGINT
@@ -20,4 +20,4 @@ TimeoutStopSec=30
 LimitMEMLOCK=infinity
 
 [Install]
-WantedBy=multi-user.target
+WantedBy=multi-user.target