ewaol-distro: Add additional user accounts into EWAOL images

This patch brings the following changes:
 * Adds following packages into all EWAOL images: 'sudo', 'bash'
   and 'bash-completion'.
 * Sets up two user accounts:
   - 'user' without administrative privileges,
   - 'ewaol' with administrative privileges via the 'sudo' group.

Issue-Id: SCM-3905
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Change-Id: Icb0ad09ef34ea93a2627d433efd0699f9a00be51