boost the devstack jobs to timeout at 90 minutes

rax nodes are slow at the moment, and have been killing off jobs
because they are taking too long to run. Unfortunately because of
the way that jenkins kills them off, we have no idea what's really
going on.

Up the timeout to 90 minutes in the hopes of figuring this out.

Fixes-Bug: 1243449

Change-Id: I83429a95c0587eae6c56dbf2863b41e815d82a4d