get_compute_usage currently requires a user to provide a datetime
object. That's actually fairly unfriendly. Also, it turns out if you
happen to produce one of those with timezone offset information, Nova
will return a 400 error. It's pretty easy to convert a datetime with
timezone info to a datetime in UTC without it - so do that first.
Now a user can submit a fully compliant ISO 8601 datetime with whatever
info they want and we'll make it meet nova's standards. They can also
supply a datetime if they prefer.
Also - do date parsing before project get - to avoid an API call if the
validation that doesn't need an API call fails.
Finally, supply a default for 'start' of the first day OpenStack
existed.
Change-Id: I53989d47f0d24695c19c1023e35acb315bec9eea
072001a628