.security index has dynamic mappings which can lead to errors

[honzakral]

Elasticsearch version : 6.x, master

Description of the problem including expected versus actual behavior: settings for the .security index are set to dynamic: true for rules and metadata which can cause conflicts and also cause mapping explosion in case of many rules.

Steps to reproduce:

1. create role mapping:

{"enabled": true, "roles": ["superuser"], "rules": {"field": {"metadata.saml": 12}}}

2. attempt to create another role mapping:

{"enabled": true, "roles": ["superuser"], "rules": {"field": {"metadata.saml": "X"}}}

this will now fail since a mapping has been created for rules.field.metadata.saml in the .security index.

this is explicitly enabled in https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/core/src/main/resources/security-index-template.json#L154

the dynamic value should be set to false for rules in the template

[elasticmachine]

Pinging @elastic/es-security