Log pre and post test hooks to file and stdout.

* devstack-vm-gate-wrap.sh: Redirect stderr into stdout, pipe the whole
thing through tee to log to a file and stdout when running the pre and
post test hooks in devstack gate.

Change-Id: Iba86ad3f5f64264cd5670248ba18422a0afd0efe
Merge "Log pre and post test hooks to file and stdout."