Refactor documentation for authentication options for MMS Monitoring and Backup Agents

Until recently, we supported no auth, username/password (MONGODB-CR) and Kerberos.

No auth and username/password auth are so obvious that they didn't really need much documentation and Kerberos is not widely used and so was documented off on its own page:

http://mms.mongodb.com/help/tutorial/connect-to-hosts-with-kerberos-authentication/#agent-directory

We now support LDAP auth as well. I think we should roll this all into one nice parent page "Agent Authentication Options".

The LDAP section should include DOCS-3819 and the Kerberos section should correct DOCS-3857.

CC cory.mintz@10gen.com