Add flavor name validation when create flavor

Check flavor name when create flavor, raise bad request instead
of internal error when flavor name isn't present in request.

Change flavor unit tests to make flavor create request body
validation being tested.

Closes-Bug: 1220087

Co-authored-by: Takashi Natsume <natsume.takashi@lab.ntt.co.jp>
Change-Id: I3ec1642305590aff61e54252405420082ac2f596
Merge "Add flavor name validation when create flavor"