Fixes bug #1045456
The date range in Nova's os-simple-tenant-usage is expected to be in UTC
time since launch/termination dates are stored in the DB in UTC time and
we use the client supplied parameters to query DB without conversion.
Switch from using datetime.today() to datetime.utcnow() to fix the issue.
Add a test for the default date range.
Import timeutils from openstack-common so we can control the return value
of utcnow() in the tests.
Change-Id: Iac77e3a4cc9561714d1492c54cef931f9764531e
f2d2e4cb06