Update development documentation about Astute installation

* Update Astute part (Use Ruby 2.1 instead of 1.9)
* Update building iso part (Use Docker)

Change-Id: I32ed55344abe6d6432dd7e6c889ef127c5a0e65f
Implements: blueprint fuel-new-system-ruby-in-master-node
Implements: blueprint fuel-containerization-of-services
Merge "Update development documentation about Astute installation"