Merge unit tests of "create a flavor" API

In test_flavor_manage, there are success/bad_request tests of
"create a flavor". Most parts of them are duplicated.
This patch merges them for the readability and clarifying their
purposes.

Change-Id: Ic7fa319674594f4470b6eafd3e07e7c467b99477
Merge "Merge unit tests of "create a flavor" API"