Supply a timeout value to devstack-gate

Increase the Jenkins timeout by 5 minutes, and supply the intended
timeout value to devstack-gate as an env variable.  This will allow
devstack-gate to timeout the gate job and have 5 minutes to copy
logs.

Change-Id: Iac0475e8c885274a3ff41adb4affb1499680a87e
Merge "Supply a timeout value to devstack-gate"