Improve terraform code

What does this MR do?

It decouples remote server provisioning and the testing using the provisioned servers.

The server provisioning jobs are manual, so that it is not necessary to be triggered for a re-run of test.

However, since the cloud resources may not be stable due to factors like resource limits, unstable networks, internal implementation bugs of the program under test, it sometimes require re-provisioning of servers. The fine-grained manually triggered provisioning jobs in this MR provides such functionality.