Support installing with Python 3 on CentOS 7

RHEL/CentOS 8 dropped support for some older hardware that is still
in use. Since CentOS 7 has Python 3, let's support it for now.

Also fixes an issue with C.UTF-8 locale which does not seem to work on
CentOS 7 (and actually causes a failure with newer python packages).

Change-Id: I1b5797b030ef896ad4b2a95a504a0215ca6ee574

.zuul.yaml

@@ -79,6 +79,15 @@
       image_type: 'dib'
       image_distro: 'centos7'
 
+- job:
+    name: ironic-python-agent-check-image-dib-centos7-python3
+    parent: ironic-python-agent-check-image-base
+    required-projects:
+      - openstack/diskimage-builder
+    vars:
+      image_type: 'dib'
+      image_distro: 'centos7'
+
 - job:
     name: ironic-python-agent-check-image-dib-centos8
     parent: ironic-python-agent-check-image-base
@@ -133,6 +142,8 @@
         - ironic-python-agent-check-image-dib-centos8
         - ironic-python-agent-check-image-dib-centos7
         # Non-voting jobs
+        - ironic-python-agent-check-image-dib-centos7-python3:
+            voting: false
         - ironic-python-agent-check-image-dib-fedora:
             voting: false
         - ironic-python-agent-check-image-dib-debian:

dib/ironic-python-agent-ramdisk/install.d/ironic-python-agent-ramdisk-source-install/60-ironic-python-agent-ramdisk-install

@@ -13,11 +13,28 @@ IPADIR=/tmp/ironic-python-agent
 UPPER_CONSTRAINTS=/tmp/requirements/upper-constraints.txt
 VENVDIR=/opt/ironic-python-agent
 
+IPA_PYTHON_VERSION=$DIB_PYTHON_VERSION
+IPA_PYTHON="$DIB_PYTHON"
+
+case "$DISTRO_NAME" in
+    centos7|rhel7)
+        # NOTE(dtantsur): C.UTF-8 doesn't seem to exist in CentOS 7
+        export LC_ALL=en_US.UTF-8
+
+        if grep -q 'Python :: 3 :: Only' $IPADIR/setup.cfg; then
+            echo "WARNING: using Python 3 on CentOS 7, this is not recommended"
+            ${YUM:-yum} install -y python3 python3-devel
+            IPA_PYTHON=python3
+            IPA_PYTHON_VERSION=3
+        fi
+        ;;
+esac
+
 # create the virtual environment using the default python
-if [ $DIB_PYTHON_VERSION == 3 ]; then
-  $DIB_PYTHON -m venv $VENVDIR
+if [ $IPA_PYTHON_VERSION == 3 ]; then
+  $IPA_PYTHON -m venv $VENVDIR
 else
-  $DIB_PYTHON -m virtualenv $VENVDIR
+  $IPA_PYTHON -m virtualenv $VENVDIR
 fi
 
 # pip might be an older version which does not support the -c option, therefore

releasenotes/notes/centos7-python3-185f1d35c37096c7.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Supports building images for Ussuri+ on CentOS 7 using Python 3. This is
+    not recommended but is necessary for some older hardware.