This lays the groundwork for better support of SELinux on platforms

that support it. SELinux rules are based in large part on

filesystem location. By installing sites into a default path, we

can create the SELinux rules on those standard paths.
This patch also moves a few global variables into a new platform.py

file. This is so that packagers for Linux distributions have an

easy place to identify things like filesystem paths that commonly

need to be modified to meet the system's packaging guidelines.
NOTE: This patch changes the current behavior in one significant

place: rb-site can no longer reference relative paths for the site

directory. Any site input that is not absolute will be assumed to

be relative to the default site path instead of the current

directory.

I created a site using 'rb-site install rb.example.com', which succeeded. I then also tested 'rb-site upgrade rb.example.com', which also succeeded.