Enable native realm in addition to the file realm

[charith-elastic]

It seems that the native realm is only enabled by default if no other realms have been defined. In #1698 we explicitly enabled the file realm -- which results in the native realm getting disabled and preventing users from authenticating using the credentials they create using the UI. See #2036.

[eedugon]

For reference purposes, there's a valid workaround explained here:
https://discuss.elastic.co/t/new-user-cant-login-kibana/204810/2

It consists of explicitly enabling the native realm.

[ransoor2]

I did the workaround. Now I'm able to log in, but I get "Error in visualization [esaggs] > Internal Server Error" when trying to access Kibana dashboards.
Elastic version 7.4.2

[sebgl]

@ransoor2 this looks unrelated to the native realm issue. Please check both Kibana and Elasticsearch logs and create a new issue or discuss topic where appropriate.