nfvbenchvm: add 2 debug features to build-image.sh

nfvbenchvm: add 2 debug features to build-image.sh

- The new option -t (enable debug traces) allows to show in the build
  log the commands run in the shell scripts, including the commands
  defined in the disk image builder elements.

- The new option -d (debug) instructs disk-image-create to drop the
  developer in a shell inside the chroot in case an error occurs.  This
  makes troubleshooting of the image possible (inspect files, run
  commands, ...)

Change-Id: I5f76e7dec64fc4c86b1515f6b81a16e11e03eadf
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>