[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)

defaults/main.yml

@@ -1,12 +1,11 @@
 ---
-es_major_version: "6.x"
-es_version: "6.6.0"
+es_version: "7.0.1"
 es_use_snapshot_release: false
 es_enable_xpack: true
 es_package_name: "elasticsearch"
 es_version_lock: false
 es_use_repository: true
-es_templates_fileglob: "files/templates/*.json"
+es_templates_fileglob: "files/templates-{{ es_major_version }}/*.json"
 es_repo_base: "https://artifacts.elastic.co"
 es_apt_key: "{{ es_repo_base }}/GPG-KEY-elasticsearch"
 es_apt_url: "deb {{ es_repo_base }}/packages/{{ es_repo_name }}/apt stable main"