dracut: Install commands required for vdev_id

The vdev_id script requires awk, grep, and head. Use dracut_install to
ensure that these commands are available in the initrd environment.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Karsten Kretschmer <kkretschmer@gmail.com>
Closes #6443
Closes #6452