Configuration documentation updates

1. Add < code > to configuration options
2. List env variables in tabular format to be consistent with other pages.
3. Moved Viewing Spark Properties section up.

This is against branch-1.0, but should be cherry picked into master as well.

Author: Reynold Xin <rxin@apache.org>

Closes #851 from rxin/doc-config and squashes the following commits:

28ac0d3 [Reynold Xin] Add <code> to configuration options, and list env variables in a table.