Fix bug 1703 - Adding non-root user to run NBI

The idea behind this is to run NBI as a non-root user for security
reasons. This required the creation of a new user in the container
image and setting permissions acordingly.

Change-Id: I9c8105b9b648c79643940994e51815118b9bbbe7
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
(cherry picked from commit a8e75d03)