Updates to installation instructions.

Staged at:
http://stg-web101.sw.joyent.continuuity.net/cdap/2.5.2-r2.5.2_installation_fix/en/admin-manual/installation/installation.html
It seems like this section is too focused on a couple things, and there is a lot of general stuff missing here.  There should be an explanation about cdap-site.xml, creating your own config with update-alternatives (didn't we used to have this?), and a link to our appendix with all the various cdap-site.xml entries.  Then I would think the current content should be moved into subsections (configuring security, changing the exposed endpoints).