Update DevStack guide when querying the image UUID

The command to query the UUID of the default image to be deployed
was retuning more than one UUID because of a grep problem, that would
also cause the "nova boot" command to fail because it was passing more
arguments that expected.

This commit fix the regex to query the UUID of the image by ignoring
what comes after the name of the image.

Change-Id: I03ad610fdd3313977b61d0d82bd1b33811f7ece1
Merge "Update DevStack guide when querying the image UUID"