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/defaults/docker_images.yml b/defaults/docker_images.yml
index bd97ea5..749133b 100644
--- a/defaults/docker_images.yml
+++ b/defaults/docker_images.yml
@@ -12,7 +12,7 @@
     ###
     # phpldapadmin:0.6.12
     docker_image_phpldapadmin: "${_param:mcp_docker_registry}/mirantis/cicd/phpldapadmin:${_param:mcp_version}"
-    # gerrit:2.13.6
+    # gerrit:2.15.17
     docker_image_gerrit: "${_param:mcp_docker_registry}/mirantis/cicd/gerrit:${_param:mcp_version}"
     # mysql:5.6
     docker_image_mysql: "${_param:mcp_docker_registry}/mirantis/cicd/mysql:${_param:mcp_version}"