[7.x] add support for elasticsearch 7.x and remove support for 5.x (#558)

- add support for elasticsearch 7.x
- remove support for elasticsearch 5.x
- update kitchen-ansible configuration (install ansible and jmespath dependencies using os repositories)
- replace geoip plugin in tests as this one is now embeded in elasticsearch since 6.7.0 (cf. https://www.elastic.co/guide/en/elasticsearch/plugins/6.7/ingest-geoip.html)
- update discovery configuration for 7.x (in ES 7.x, discovery.zen.ping.unicast.hosts is replaced by discovery.seed_hosts and transport.tcp.port is replaced by transport.port, also discovery.seed_hosts is disabled on master nodes to avoid "master_not_discovered_exception" error when creating templates in the same play)
- update index template structure for 7.x 
- update security realms settings for 7.x (cf. https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes-7.0.html#include-realm-type-in-setting)

files/templates-6.x/basic.json

@@ -0,0 +1,11 @@
+{
+  "template" : "te*",
+  "settings" : {
+    "number_of_shards" : 1
+  },
+  "mappings" : {
+    "type1" : {
+      "_source" : { "enabled" : false }
+    }
+  }
+}