Run weston with dedicated 'display' user and group

 * Create a user/group display
 * Allow weston to start without mandatory root user
 * start weston-terminal for each user

Bug-AGL: SPEC-546

Change-Id: Id50acdbf5f7c07d5e0440575d42998b8819b5547
Signed-off-by: Ronan Le Martret <ronan.lemartret@iot.bzh>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9135
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Dominig ar Foll <dominig.arfoll@fridu.net>
Reviewed-by: José Bollo <jobol@nonadev.net>
Reviewed-by: Stéphane Desneux <stephane.desneux@iot.bzh>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>