With the introduction of centos 8 we have constructs like
if [[ $DISTRO =~ (centos|fedora) && $DIB_RELEASE -ge 8 ]]
This is intended to match the "centos7" element (from the =~) but it
was missed that this is setting the DIB_RELEASE to "GenericCloud".
I think it makes more sense for this to be a numeric release, and
makes constructs like above work. There really isn't any other type
of image to choose here; thus we move it into a new, centos7
specific variable.
Note that when the centos 8 images are available, we want to move to a
generic "centos" element that will handle both 7 and 8 together (same
as rhel) based on DIB_RELEASE and deprecate centos7; this works with
that environment too.
Change-Id: I2e6b7848070d6452c0563e2a122447627c6e6bf7
9f688f53da