Fix FIP negative tests for nova V2.1 API

Nova V2.1 API have strong input validation.
Ia04256e0c1f9235769035cc9e7eadd864112559c adds schema for add floating ip
for strong input validation but tempest negative test fail.

In case of empty FIP, tests should expect Bad Request also in case
of V2.1.

Closes-Bug: #1414996

Change-Id: I38a2f5cb336ddd22ac920aebecf95bdcd3524a74
1 file changed