[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)

test/integration/helpers/serverspec/xpack_spec.rb

@@ -4,11 +4,7 @@ shared_examples 'xpack::init' do |vars|
   describe file("/etc/elasticsearch/#{vars['es_instance_name']}/elasticsearch.yml") do
     it { should contain "node.name: localhost-#{vars['es_instance_name']}" }
     it { should contain 'cluster.name: elasticsearch' }
-    if vars['es_major_version'] == '6.x'
-      it { should_not contain "path.conf: /etc/elasticsearch/#{vars['es_instance_name']}" }
-    else
-      it { should contain "path.conf: /etc/elasticsearch/#{vars['es_instance_name']}" }
-    end
+    it { should_not contain "path.conf: /etc/elasticsearch/#{vars['es_instance_name']}" }
     it { should contain "path.data: /var/lib/elasticsearch/localhost-#{vars['es_instance_name']}" }
     it { should contain "path.logs: /var/log/elasticsearch/localhost-#{vars['es_instance_name']}" }
     it { should contain 'xpack.security.enabled: false' }