Remove prepare_instance_network in ImagesOneServerNegativeTestJSON

In ImagesOneServerNegativeTestJSON, all testcases are negative and
have no relationship with "ssh into the server", so there is no
need to use prepare_instance_network in setup_credentials. This is
to remove it for code clean.

Change-Id: I7de581e19ff9eb02473dd2f26bb833e2d270fd06
1 file changed