In devstack, the exercises/euca.sh test the "VOLUME". it should always
check the volume which was just created. Because there may be volumes
left by the last failed euca.sh, then the variable VOLUME
VOLUME=`euca-describe-volumes | cut -f2`
will be assigned a list of volumes, which is not expected.
Fixes bug #1191424
Change-Id: Ie004c9cfeaef4a8a313866d21d45c7b41f1b2ba0