Fix flavor swap default value for compute microversion 2.75

As of microversion 2.75 default return value of swap in the response of
create_flavor is 0 instead of empty string, so this is to fix
test_create_list_flavor_without_extra_data for microversion 2.75.

https://docs.openstack.org/api-ref/compute/?expanded=create-flavor-detail#create-flavor

Change-Id: Ib086f9977f838cf884c7324b7b6eead698adaa9e
1 file changed