install: enable xpaas install experience

https://trello.com/c/5e9f2ejA/275-5-xpaas-preparedness

Changes to openshift.sh so that when the fuse cartridge is released, we
have an "xpaas" profile that can work with it out of the box.

This also makes some generalized changes:
* Introduce a setting for proxy ports per gear
* Introduce a setting for district UID range
* The SNI proxy is installed, configured, and firewall ports opened
* Introduce settings for SNI proxy port range

Then, when a node is designated with node_profile=xpaas:
* External ports per gear defaults to 15
* The SNI proxy defaults to listening at 10 ports
* xpaas districts are created with reduced capacity (to match 15 ports)
install: organize header comments

The comments, particularly the parameter listing, had grown somewhat out
of control. They are now organized into hopefully-logical sections with
a table of contents.

No code changes were made.