Decrease amount of threads for Tempest tests

There are cases when Cinder volume quota is exceeded with
high load in gate tests with generic driver, that uses Cinder.

Until we have proper way to increase Cinder volume quota, amount
of threads is decreased for better stability of gate tests.

Change-Id: I2c74aee51aa6c1c3cb4fb4ccd9f28d5a05290b52
Merge "Decrease amount of threads for Tempest tests"