Separate negative tests for list_floating_ips

Add negative tests for test_list_floating_ips to a specific file.
Since no floating_ips are needed to setup for negative test,
generation of non existent id can be simplified.

bp negative-test-files

Change-Id: I1d3ac0956ea3f69bbd5bde897f0a18fc0e182930
Separate negative tests for test_fixed_ips

Move negative tests from test_fixed_ips.py to test_fixed_ips_negative.py

Partially implements blueprint negative-test-files
Change-Id: I965b85790463eb5de7c77d237996e469b0c4423f
Merge "Separate negative tests for list_floating_ips"
Merge "Separate negative tests for test_fixed_ips"