run_stress.py -h doesn't work without a connection

run_stress.py tries to connect to openstack even though it's just a
--help / -h call. Just moved the import to the corresponding function.

Implements: blueprint stress-tests

Change-Id: Id0f9784a64f3bf53ea91a67452a78d17389d7f51
Merge "run_stress.py -h doesn't work without a connection"