Use correct devstack dib element with centos7

The node-devstack element was misspelled under the centos7 nodepool
diskimage. Fix the typo so that we can build centos7 images this way.

Change-Id: I7d2c39c4a91e04d20309d6c4ef1ddfbc92784f51
Merge "Use correct devstack dib element with centos7"