Fix 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: Idf967204b19818ad623c67a19c134cd78a97b9dc
Signed-off-by: beierlm <mark.beierl@canonical.com>