Use Centos 7 cloud image symlink
Use Centos 7 cloud image symlink

We should make use of the CentOS-7-x86_64-GenericCloud.qcow2 symlink from
http://cloud.centos.org/centos/7/images/ instead of having a hard coded cloud
image. Specific cloud images can still be downloaded by overriding
$DIB_RELEASE.

More importantly, using the symlink will keep us automatically up to date with
the latest CentOS 7 cloud image. The image in use by the hard coded value
occassionally exhibits "No space left on device" errors after the cloud-init
filesystem resize. More info about this issue is at:
http://xfs.org/index.php/XFS_FAQ#Q:_Why_do_I_receive_No_space_left_on_device_after_xfs_growfs.3F

The newer cloud image (with a newer kernel) does not exhibit this issue.

Change-Id: I3e19f6269ceba937fcd630bab265d132bd525519

zioproto/diskimage-builder,openstack/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,rdo-management/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,switch-ch/diskimage-builder,rdo-management/diskimage-builder