Allow an override for the default timeout

There's a hardcoded default timeout of 900 seconds.  This timeout is used when warming up the inactive stack (waiting for the desired number of instances to be verified as healthy).  If you have a component with a large number of instances, 900 seconds may not be long enough to verify all instances healthy - the deployment will then fail, even though the operation on Amazon's side will complete successfully.

Allow the default timeout to be overridden, to account for natural variations in warm up times for components.