Fix DevStack setup on CentOS 8.3

CentOS 8.3 changed the name of the PowerTools repository to powertools:
https://wiki.centos.org/Manuals/ReleaseNotes/CentOS8.2011#Yum_repo_file_and_repoid_changes

With this repository disabled, DevStack fails to install libyaml-devel,
which causes a failure to install many packages. In my environment
DevStack stopped with an error caused by a missing wget.

Keep the command using the old repository name, for compatibility with
older CentOS releases.

Change-Id: I5541a8aee8467abf10ce8a10d770618bdd693f02
diff --git a/stack.sh b/stack.sh
index d3c1476..6375c8e 100755
--- a/stack.sh
+++ b/stack.sh
@@ -365,6 +365,9 @@
     # EPEL packages assume that the PowerTools repository is enable.
     sudo dnf config-manager --set-enabled PowerTools
 
+    # CentOS 8.3 changed the repository name to lower case.
+    sudo dnf config-manager --set-enabled powertools
+
     if [[ ${SKIP_EPEL_INSTALL} != True ]]; then
         _install_epel
     fi