Place the generated list in the correct location for the proposal job

If tools/generate-devstack-plugins-list.sh is invoked with a parameter,
as is the case with the proposal job, copy the generated list thither
so that git will actually generate a diff.

Change-Id: I874a521bba7f402f5c07e28d6ebb086e77873795
diff --git a/tools/generate-devstack-plugins-list.sh b/tools/generate-devstack-plugins-list.sh
index 3710522..b7817a0 100644
--- a/tools/generate-devstack-plugins-list.sh
+++ b/tools/generate-devstack-plugins-list.sh
@@ -54,3 +54,7 @@
 
 test -r data/devstack-plugins-registry.footer && cat data/devstack-plugins-registry.footer
 ) > doc/source/plugin-registry.rst
+
+if [[ -n ${1} ]]; then
+    cp doc/source/plugin-registry.rst ${1}/doc/source/plugin-registry.rst
+fi