Up the default timeout for stack builds

This commit ups the default stack build timeout from 600 to 1200.
The stack frequently can take longer than 600 secs to build so
doubling it should give us a safe margin.

Related-Bug: #1297560

Change-Id: I3b13a225793d42e2fa9f4871b2fe81c745954c56
Merge "Up the default timeout for stack builds"