Flavor name should not contain only white spaces

Returns 400 Bad request instead of 200 Ok if flavor name
contains only whitespaces.
Flavor name containing any leading and trailing whitespaces will be
trimmed before validation and before saving it to the backend.

Added unit test cases for both v2 and v3 api's. For v3 api instead of
stubbing flavors 'create' method stubbed db's 'flavor_create' method.
Also removed redundant code of stubbing 'flavor_create' method.

For example
name = "  "              #not allowed
name = "extra large"     #allowed
name = " extra large "   #allowed, but leading and trailing whitespaces
                         #will be trimmed (same as instance name)

Closes-Bug: #1237611

Change-Id: Ie68be299030a878a752dbfdcdfe5094f8427598a
Merge "Flavor name should not contain only white spaces"