CLOUDSTACK-6047: Make aggregation command timeout configurable

In case some environments has different performance or we found some commands
would took too long to execute, one global configuration item is introduced to
specify "time out in seconds per one command in aggregation commands".

By default it's 3 seconds. If admin feel it's too long, it can be adjust to as
low as 1 seconds, which runs still well in my machine.

Conflicts:
	setup/db/db/schema-430to440.sql