Fix "fixed_ip" parameters in unit tests

In test_create_instance_with_neutronv2_fixed_ip_already_in_use of both
v2 and v3, these tests pass wrong parameters "fixed-ip" instead of
valid parameter "fixed_ip".
The purposes of these tests are to check the behaviors when passing
in-use addresses, but current tests contain two negative factors and
we cannot test the purposes now.

This patch fixes the parameters. This problem was detected through the
API validation works and Ic17d2227909c9180ca86c9d578dad4c73ec57dfc needs
this fixing.

Change-Id: I4d54a1522983c996431b995ed157b32d13c3bc88
Closes-Bug: #1331317
Merge "Fix "fixed_ip" parameters in unit tests"