Add a minimal gate_hook.sh script

This will ensure that, even after I0c44229b2f67c9ea50e3f8786dccd8c50eb8e5e3
lands, our DevStack gate job will continue to pass.

Change-Id: I63ec82bf5b6ee2c6b58608676ed6709adec6fab2
Merge "Add a minimal gate_hook.sh script"