Raise orchestration build_timeout to 600 seconds

Observed boot time for a single server has been around 250 seconds
so a build_timeout default of 300 would explain why ~20% of heat-slow
jobs are failing with stack timeout errors.

Closes-Bug: #1288970
Change-Id: I4feb1b89acf8db0e164468d0471aff71ff5c6a77
Merge "Raise orchestration build_timeout to 600 seconds"