daemon: Only show user roles that actually exist

Reviewed-by: Stef Walter <stefw@redhat.com>
daemon: Add "docker" user role

Fixes #1207
Reviewed-by: Stef Walter <stefw@redhat.com>
daemon: Update roles when /etc/group changes

Reviewed-by: Stef Walter <stefw@redhat.com>
test: Check dynamic updates of role groups

Closes #1235
Reviewed-by: Stef Walter <stefw@redhat.com>