Clarified some steps of the installation process

Fixes bug 1192630

Changed the title of the instruction regarding the user creation, as
users with other names run into bugs.
Changed the bash $ symbol for some commands to # where commands are
ran as a root user
Corrected syntax for NOPASSWD in sudoers
Added ip tables rule to allow guest access to the internet

Change-Id: Iffa402fa2d717b53dab01bbefcc10943d05d6169
Merge "Clarified some steps of the installation process"