Hitachi: test script fails unexpectedly

[Atsushi Kawai]

[Overview]
Hitachi Cinder driver submits a REST API frequently to avoid a session timeout by a REST API server. When running test scripts for the driver, especially by ``nosetests`` command with multiple test scripts, it is some possibility of failing some test case by an unexpected response from a pseudo REST API server in the scripts because a resource of the server may be spent by the frequently REST API requests.

The scripts have codes to stop frequently REST API requests, but one of the code does not work because wrong variable name is used.

[Customer impact by the bug]
Non, because it is only issued when running test scripts

[which codes should be fixed]
--- in test_do_setup() ---
# wrong
self.driver.common.client.keep_session_loop.stop()

# correct
drv.common.client.keep_session_loop.stop()

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/cinder/+/916724