Make test_poll_volume_usage_with_data more reliable

This test includes 2 calls to time.time().  It assumes that the result
from the second call may be greater than the first.  This isn't
definietly true and we see periodic failures of this test in the gate as
a result.

Change the test to not worry about the specific time value that gets
set.  We set _last_vol_usage_poll to 0 before calling the function under
test.  We really only need to check that it got set to some value after
calling the function under test.  This tweak should make the test pass
consistently.

Change-Id: I6c01d2958cf671c29622562689a15ca1ddea0a3c
Closes-bug: #1270693
Merge "Make test_poll_volume_usage_with_data more reliable"