Fix devstack python detection if python3 is missing

On platforms without python3 installed devstack fails the python
version check. This does it more gracefully.

Change-Id: I4d79a41eb2d66852ab1a1aa8bd383f8e3e89cdc0
diff --git a/functions-common b/functions-common
index deebdec..b9bf0c9 100644
--- a/functions-common
+++ b/functions-common
@@ -2080,7 +2080,10 @@
 function _get_python_version {
     local interp=$1
     local version
-    version=$($interp -c 'import sys; print("%s.%s" % sys.version_info[0:2])')
+    # disable erroring out here, otherwise if python 3 doesn't exist we fail hard.
+    if [[ -x $(which $interp) ]]; then
+        version=$($interp -c 'import sys; print("%s.%s" % sys.version_info[0:2])')
+    fi
     echo ${version}
 }