Correction of the devops.rst

1. Determine common steps for all approaches and put them into as
pre installation requirements
2. Add the package repository to the installation from packages approach
3. Add some corrections and remove useless items

Change-Id: I2d97a3a09d29f05dcc060b14907e202f3aeb35d1
Closes-Bug: 1376275
Merge "Correction of the devops.rst"