Fix test_create_volume_flow test issue

Fix test_create_volume_flow test issue

One of the tests in
cinder/tests/unit/volume/flows/test_create_volume_flow.py incorrectly
mocks time.time function which means that when there's a call to
time.time() it will raise an error because real time function doesn't
expect any arguments but the mock method does expect self as an
argument.

For some reason time.time function does not get called on every test
run, but when it does get called it will fail.

This patch fixes this by binding the mock method to the current self
instance.

Change-Id: Ia70e51719abb9a6ed357802446847ffa81d7427e