Add execution bit to generate-devstack-plugins-list.sh

This commit adds an execution bit to generate-devstack-plugins-list.sh.
This should be useful for users.

Change-Id: I12d0a257eb1d487979d044c2e52e824a6ea4c02d
diff --git a/tools/generate-devstack-plugins-list.sh b/tools/generate-devstack-plugins-list.sh
old mode 100644
new mode 100755