Changes

centos8-repo-install-test: Upstream requires 'powertools' not 'PowerTools'

I just confirmed: With a centos:centos8 base image of a few weeks ago,
"PowerTools" works. With the current docker base image for
centos:centos8, only lowercase "powertools" works, uppercase is no
longer accepted.

https://access.redhat.com/discussions/5417621
https://www.reddit.com/r/CentOS/comments/jd7x3d/how_to_enable_powertools_in_centos_stream/
https://git.centos.org/rpms/centos-repos/blob/c8/f/SOURCES/CentOS-Stream-PowerTools.repo#_11

Change-Id: I0db6c82576c5aae94531b7ec2fd98429cab702de
Closes: SYS#5244