Add DIB_YUM_REPO_PACKAGE as an alternative to DIB_YUM_REPO_CONF

A custom yum repository can now be configured by defining
`DIB_YUM_REPO_PACKAGE` as a yum available package or a URL to an rpm file.
This package can install repo files with any associated keys and
certificates.

A good example of such a package upstream is rdo-release[1] which
includes multiple repo files, the repo keys, and a root certificate.
This makes these repos impractical to install via DIB_YUM_REPO_CONF.

Downstream, repo packages like this a frequently used to bootstrap
development builds of RHEL with development repos.

[1] https://www.rdoproject.org/repos/rdo-release.rpm

Change-Id: I2832e723998c9bd7635cdf7541a4c20eff6294d2

diskimage_builder/elements/rhel/environment.d/11-yum-dnf.bash

@@ -1,7 +1,7 @@
 # since RHEL8, dnf is the yum replacement.
 
-if [[ ${DIB_RELEASE} == '8' ]]; then
+if [[ $DIB_RELEASE == "7" ]]; then
-    export YUM=dnf
-elif [[ ${DIB_RELEASE} == '7' ]]; then
     export YUM=yum
+else
+    export YUM=dnf
 fi

diskimage_builder/elements/yum/README.rst

@@ -18,6 +18,10 @@ files will be copied to /etc/yum.repos.d/ during the image build, and then
 removed at the end of the build. Each repo file should be named differently to
 avoid a filename collision.
 
+The yum repository can also be configured by defining `DIB_YUM_REPO_PACKAGE` as
+a yum available package or a URL to an rpm file. This package can install repo
+files with any associated keys and certificates.
+
 Environment Variables for Module Selection during Image Creation
 ----------------------------------------------------------------
 The following environment variable is used to select module streams to be

diskimage_builder/elements/yum/pre-install.d/00-install-repo-package

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+if [ -n "${DIB_YUM_REPO_PACKAGE:-}" ] ; then
+    rpm -ivh ${DIB_YUM_REPO_PACKAGE}
+fi

releasenotes/notes/repo-package-20fe407795bffd01.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    A custom yum repository can now be configured by defining
+    `DIB_YUM_REPO_PACKAGE` as a yum available package or a URL to an rpm file.
+    This package can install repo files with any associated keys and
+    certificates.