[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)

tasks/elasticsearch-parameters.yml

@@ -12,14 +12,6 @@
   debug: msg="WARNING - It is recommended you specify the parameter 'http.port'"
   when: es_config['http.port'] is not defined
 
-- name: debug message
-  debug: msg="WARNING - It is recommended you specify the parameter 'transport.tcp.port'"
-  when: es_config['transport.tcp.port'] is not defined
-
-- name: debug message
-  debug: msg="WARNING - It is recommended you specify the parameter 'discovery.zen.ping.unicast.hosts'"
-  when: es_config['discovery.zen.ping.unicast.hosts'] is not defined
-
 #If the user attempts to lock memory they must specify a heap size
 - name: fail when heap size is not specified when using memory lock
   fail: msg="If locking memory with bootstrap.memory_lock a heap size must be specified"