Fix platform check in diskimage-create.sh
Fix platform check in diskimage-create.sh

'set -e' causes the script to exit when 'grep CentOS' fails, which
meant the subsequent test was never reached. the subsequent test was
also backwards, using -z (test for zero length) instead of -n (test
for non-zero length).

Change-Id: Ie894a0ef4d785af6b9cb4a9f690bc6752092e655

openstack/sahara-image-elements,bigfootproject/sahara-image-elements,crobby/sahara-image-elements,uladz/sahara-image-elements,openstack/sahara-image-elements,bigfootproject/sahara-image-elements,uladz/sahara-image-elements,crobby/sahara-image-elements