Change v3 flavor_manage API to v2.1

This patch changes v3 flavor_manage API to v2.1 and makes v2 unit tests
share between v2 and v2.1.
This patch removes test_create_flavor_name_with_leading_trailing_whitespace
because v2.1 API contains consistent name rule for whole of API request and
the name which contain spaces at the start/end of name is against the rule.

The differences between v2 and v3 are described on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3 .

Partially implements blueprint v2-on-v3-api

Change-Id: I57ca2562cb02ce5a35c4596e4c5a753d1268493f
Merge "Change v3 flavor_manage API to v2.1"