Refactor deprecated filter calls

Previously, each of the calls generated a warning : "Using tests as
filters is deprecated. Instead of using `result|version_compare` use
`result is version_compare`. This feature will be removed in version
2.9.".

templates/systemd/elasticsearch.j2

@@ -18,13 +18,13 @@ WorkingDirectory={{es_home}}
 User={{es_user}}
 Group={{es_group}}
 
-{% if (es_version | version_compare('6.0.0', '<')) %}
+{% if (es_version is version_compare('6.0.0', '<')) %}
 ExecStartPre=/usr/share/elasticsearch/bin/elasticsearch-systemd-pre-exec
 {% endif %}
 
 ExecStart={{es_home}}/bin/elasticsearch \
                                     -p ${PID_DIR}/elasticsearch.pid \
-{% if (es_version | version_compare('6.0.0', '<')) %}
+{% if (es_version is version_compare('6.0.0', '<')) %}
                                     -Edefault.path.logs=${LOG_DIR} \
                                     -Edefault.path.data=${DATA_DIR} \
                                     -Edefault.path.conf=${CONF_DIR} \