* 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
