Updating the setup development environment docs for icehouse

Changes
* rewording title paragraph to be more descriptive of the document
* adding a link to devstack.org in the "... inside DevStack" section
* adding a sentence about developing on OSX to the "... inside DevStack"
section
* changing the toctree link to the OSX VM install doc to be inline
* minor grammar cleanup on section titles
* adding a header paragraph to the ui dev environ page
* rewording the "... as part of DevStack" section to be similar to dev
environ doc
* minor cleanups to ui dev environ page
* correcting sahara-dashboard location in /opt/stack

Partial-Implements: blueprint update-docs-icehouse

Change-Id: Ideed96512ee7f28280577f2520bacc620952e305
Merge "Updating the setup development environment docs for icehouse"