Using execCmd to run dmidecode command instead of using subprocess.Popen

This change is due to a bug while running 'vdsm-tool vdsm-id' as user
root which calls this function and fails on "require tty" for user root.

execCmd skip the sudo=True parameter while executing the command [1]

[1] http://gerrit.ovirt.org/#/c/23721/

Change-Id: I28f4aad240711a2ab4339c7551aa1d88913f735b
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1054759
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23722
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>