Set correct python version for non-chroot scripts

Some phases of diskimage-builder run outside the chroot environment,
such as the extra-data.d scripts, and don't have access to dib-python.
This means these scripts may choose the wrong python version by using
"#!/usr/bin/env python" to execute. The svc-map element is an example.

This patch creates a temporary directory and symbolic link for the
correct version of python, then manipulates the environment PATH
to preference the symbolic link "python" command.
This will allow elements with these scripts to work correctly with
the version of python diskimage-builder is running under.

Change-Id: I289d621e1bfbba0eb174dff977d1a5c92c04e4fa
Co-Authored-By: Ian Wienand <iwienand@redhat.com>

diskimage_builder/lib/common-functions

@@ -241,6 +241,14 @@ function cleanup_image_dir () {
     rm -rf --one-file-system $TMP_IMAGE_DIR
 }
 
+function cleanup_temp_python_exec () {
+    if [ ! -z "$DIB_ORIGINAL_PATH" ]; then
+        export PATH=$DIB_ORIGINAL_PATH
+    fi
+    rm "$DIB_PYTHON_EXEC_TMP/python" || true
+    rmdir "$DIB_PYTHON_EXEC_TMP" || true
+}
+
 # Run a directory of hooks outside the target (that is, no chrooting).
 function run_d() {
     check_element

diskimage_builder/lib/disk-image-create

@@ -175,6 +175,14 @@ export DIB_DEBUG_TRACE
 export DIB_IMAGE_CACHE=${DIB_IMAGE_CACHE:-~/.cache/image-create}
 mkdir -p $DIB_IMAGE_CACHE
 
+# Setup a symbolic link to the correct python exec so that element
+# scripts running outside the chroot using "#!/usr/bin/env python" get
+# the right python version.
+export DIB_ORIGINAL_PATH=$PATH
+export DIB_PYTHON_EXEC_TMP=$(mktemp -d -t --tmpdir=$HOME .DIB_PYTHON_TMP.XXXXXXXX)
+ln -s "$DIB_PYTHON_EXEC" "$DIB_PYTHON_EXEC_TMP/python"
+export PATH=$DIB_PYTHON_EXEC_TMP:$DIB_ORIGINAL_PATH
+
 # We have a couple of critical sections (touching parts of the host
 # system or download images to common cache) that we use flock around.
 # Use this directory for lockfiles.
@@ -591,6 +599,7 @@ fi
 
 # Remove the leftovers, i.e. the temporary image directory.
 cleanup_image_dir
+cleanup_temp_python_exec
 
 # Restore fd 1&2 from the outfilter.py redirect back to the original
 # saved fd.  Note small hack that we can't really wait properly for

diskimage_builder/lib/img-functions

@@ -43,6 +43,7 @@ function cleanup () {
     dib-block-device umount
     cleanup_build_dir
     cleanup_image_dir
+    cleanup_temp_python_exec
 }
 
 # Helper function to run a command inside the chroot