changes to features.rst

changed to designed to enable a user - a before user
changed to A user may change the number
changed to If a cluster fails
front-end added a -
changed to tasks to the sahara-engine
changed to A user can set how many volumes
changed to  with the task tracker service have
changed to instances in the node group
changed to recommended to control the security group
changed to recommend using it in a production environment:

Change-Id: I76df92ad63ee14966c9040629e013d7f5839542e
Closes-Bug: #1384461