Unset CentOS image parameters that affect on another images

The parameters DIB_CLOUD_IMAGE, BASE_IMAGE_FILE and DIB_IMAGE_SIZE
should be unset after centos image creating, because these parameters can be
used for building another images (for example, when we build all images)

Change-Id: I0e63f544ac2f3cb9fcf00cafb1d897514bde9554
Closes-bug: #1324585
Merge "Unset CentOS image parameters that affect on another images"