Make unit test floating ips bulk faster

The unit test is only used to test whether the code branch is
executed with pre-defined logic, so it does make sense to create
as less as dataset if we can achieve that goal.

Currently the unit test for floating ip bulk operation in nova
create a rather a big data set to run the test. This patch adjust
it with smaller one.

the results shows (ignore the first one because env setup)
                                                   [former] [current]
test_create_bad_cidr_fail                       OK  4.27     8.30
test_create_duplicate_fail                      OK  8.01     1.20
test_create_invalid_cidr_fail                   OK  0.10     0.14
test_create_ips                                 OK  0.77     0.18
test_create_ips_pool                            OK  7.65     0.17
test_delete_ips                                 OK  10.01    0.35
test_list_ip_by_host                            OK  0.14     0.27
test_list_ips                                   OK  0.15     0.25

Change-Id: I2ee5b5186f0c88b80ed20a0dbc16af9360e28332
Merge "Make unit test floating ips bulk faster"