Replace gevent with eventlet

Eventlet is used throughout OpenStack projects. This patch changes to use
eventlet and replaces gevent. Two things are changed:
1. wsgi server 2. cassandra python driver.

Change-Id: I74e7f572180bb27266a7786ca7a8cd5e5e82b779
Implements: blueprint eventlet-transition
Add user guide

Add content to doc/source/user_guide.rst

Change-Id: Ia1b58a866abef8681a6adda7060562baaaaf4001
Adds developer guide

Add developer guide.
Add CLI client documentation.

Implements: bp magnetodb-documentation
Change-Id: I83e7e61b56e3d17c1412e609aa22d9b3f864c7dd
Merge "Adds developer guide"
Merge "Add user guide"