Fix _parse_datetime in simple tenant usage extension

_parse_datetime in os-simple-tenant-usage incorrectly attempts
to parse the datetime string even when it is None. As a result
a 400 BadRequest is returned. This has the overall effect of making
the start and end datetime parameters compulsory when they are meant
to be optional.

Note that this restores the behavior where start/end are optional which is
in contradiction to the API docs, but is the long standing API behaviour.
This was an API change which was accidentally applied in commit
I8e0e870727d687da165c809ffb7a4456bff81122 as part of some nova internal
changes but not picked up by the unitests

Change-Id: I00427379dc7aa39770f9a16ff026addb6e311735
Closes-Bug: 1300972
Merge "Fix _parse_datetime in simple tenant usage extension"