Fix systemd pidfile setting

This commit fixes the pidfile setting on systems that used systemd. The
issue is that the pidfile can only be set via the command line arguments
-p or --pidfile, and is no longer settable via a setting.

distribution/src/main/packaging/systemd/elasticsearch.service

@@ -20,7 +20,7 @@ Group=elasticsearch
 ExecStartPre=/usr/share/elasticsearch/bin/elasticsearch-systemd-pre-exec
 
 ExecStart=/usr/share/elasticsearch/bin/elasticsearch \
-                                                -Ees.pidfile=${PID_DIR}/elasticsearch.pid \
+                                                -p ${PID_DIR}/elasticsearch.pid \
                                                 -Ees.default.path.home=${ES_HOME} \
                                                 -Ees.default.path.logs=${LOG_DIR} \
                                                 -Ees.default.path.data=${DATA_DIR} \