Refactor security documentation for 2.5.2

Some refactoring for the Security documentation. Gives better distinction between the configuration and the usage.

Known issues:
1. The three tables under ``LDAP Authentication`` are formatted differently. I can't figure out how to get them to align with each other.
2. Some of the links are broken.
3. As discussed on Hipchat, we may want to consider pulling ``Security`` out one more level.
"CDAP supports securing clusters using perimeter security. Configuration and client authentication are covered in this section."