Update Gerrit image version

* update gerrit to 2019.2.6 with v2.15.17 version of Gerrit;
* set IGNORE_VERSIONCHECK var for Gerrit container, for
  full reindex data before start for upgrade [0,1]

[0] https://www.gerritcodereview.com/2.13.html#online-reindexing
[1] https://www.gerritcodereview.com/2.14.html#reindex-for-new-groups-index

Change-Id: I02924e49343ae1f2ecaa1c3409a9bf79df63120e
Related-Prod: PROD-33894
diff --git a/docker/swarm/stack/gerrit.yml b/docker/swarm/stack/gerrit.yml
index ed90acf..d1a5aa7 100644
--- a/docker/swarm/stack/gerrit.yml
+++ b/docker/swarm/stack/gerrit.yml
@@ -51,6 +51,7 @@
                 GERRIT_ADMIN_PWD: ${_param:gerrit_admin_password}
                 GERRIT_ADMIN_EMAIL: ${_param:gerrit_admin_email}
                 CANLOADINIFRAME: "true"
+                IGNORE_VERSIONCHECK: "false"
                 JAVA_OPTIONS: "-Djavax.net.ssl.trustStore=/etc/ssl/certs/java/cacerts ${_param:gerrit_extra_opts}"
                 https_proxy: ${_param:docker_https_proxy}
                 http_proxy: ${_param:docker_http_proxy}