Add GSSAPI auth and host-based auth configuration to security overview doc

[rolandcrosby]

Exalate commented:

The table of security features on the security overview page should be updated for 19.1, now that we have GSSAPI authentication and configurable authentication methods by host.

Jira Issue: DOC-266

[jseldess]

Good point, @rolandcrosby. Do you think GSSAPI should be mentioned somehow in the body of that page as well?