Use concurrent.futures instead of multiprocessing

On the environment with enabled SSL the SSLv3_ALERT_BAD_RECORD_MAC error
occurs in fio_setup/fio_cleanup modules, in the section where processes
of creating/deleting VMs are paralleled.
This is due the paralleled processes have to communicate with each other
about the state of the shared SSL connection.
Changing to multithreading resolves the issues.

Related-PROD: #PROD-37187
Change-Id: I4f15ef900c79811db0d807ae2c91779595f497d2
2 files changed