This patch adds support for CentOS 8 Stream [1] to the centos element
(cloud image). Users should set DIB_RELEASE=8-stream.
[1] https://www.centos.org/stream/
Change-Id: Ib8f542031c46326ffed812fa60cbc9e56db9d6fd
* Add "centos" element, a CentOS version-independent element. This is in
line with the same work done for RHEL in Stein cycle.
* Deprecate the centos7 element. CentOS 7 support itself it not
deprecated though. The new "centos" element provides the same support
level as the "centos7" element.
* Add functional testing
The default CentOS version is 8. You can adjust it using the DIB_RELEASE
environment variable.
Change-Id: I373ba2296c4613765676e59aabd9c651345298d1
