rhel-common: Provide method to select module streams

Some OpenStack releases on RHEL require specific modules
in order to function correctly.  This adds the ability
to set DIB_DNF_MODULE_STREAMS which then are selected
prior to package installation.

Change-Id: I78d7bcf214a45245e2073428120fcbdd968e1acd
Signed-off-by: Lon Hohberger <lhh@redhat.com>

diskimage_builder/elements/rhel-common/README.rst

@@ -24,15 +24,19 @@ Satellite.  An activation key can only be used for registration purposes using
 the subscription-manager command line tool and is considered a lower security
 risk.
 
-IMPORTANT NOTE:
+IMPORTANT NOTES:
 ----------------
 The 00-rhsm script is specific to RHEL6.  If you use the REG\_ variables to
 use with RHEL7, you do not need to set any DIB_RHSM variables.  The scripts
 named with "rhel-registration" have not been developed or tested for RHEL6.
 For information on building RHEL6 images, please see the rhel element README.
 
-Environment Variables For Image Creation
-----------------------------------------
+The 01-module-configuration script is specific to RHEL8.  RHEL8 includes
+various versions of software which can be selected during installation, some
+may be older and/or incompatible with a given release of OpenStack.
+
+Environment Variables For Regisration during Image Creation
+-----------------------------------------------------------
 The following environment variables are used for registering a RHEL instance
 with either the Red Hat Customer Portal or Satellite 6.
 
@@ -244,3 +248,20 @@ use the following metadata::
         }
     }
 
+
+Environment Variables for Module Selection during Image Creation
+----------------------------------------------------------------
+The following environment variable is used to select module streams to be
+enabled during an image build on RHEL8.  Any existing stream for the given
+module is first disabled prior to enabling the specified stream.
+
+#### DIB\_DNF\_MODULE\_STREAMS
+This is a space-separated list of module streams to enable prior to any
+RPMs being installed.
+
+Image Build Module Selection Example
+------------------------------------
+When using Train on RHEL8.2, one must select the appropriate virt and
+container-tools module streams:
+
+DIB_DNF_MODULE_STREAMS='virt:8.2 container-tools:2.0'

diskimage_builder/elements/rhel-common/pre-install.d/01-module-configuration

@@ -0,0 +1,22 @@
+#!/bin/bash
+#
+# When building RHEL overcloud images, certain modules may be preferred
+# or even required.
+#
+# For example, container-tools:2.0 is notably a requirement when running
+# the Train releases on RHEL 8.2.
+#
+
+if [ "${DIB_DEBUG_TRACE:-0}" -gt 0 ]; then
+    set -x
+fi
+
+set -eu
+set -o pipefail
+
+if [ "${YUM}" == "dnf" ]; then
+    for m in ${DIB_DNF_MODULE_STREAMS}; do
+        ${YUM} -y module disable ${m/:*/}
+        ${YUM} -y module enable ${m}
+    done
+fi

releasenotes/notes/rhel-module-selection-e9f6f06ece1fb829.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    New environment variable enables the ability to select specific module
+    streams prior to RPM installation on RHEL8 and later.