Network tests should take advantage of net_utils to find unused ip.

Currently, test_create_router_external_fixed_ips fails [0] for the
same reason described in this patch [1].

Basically, calling random.choice to select an ip address from
an allocation pool can potentially select an already-used ip
address, resulting in an error.

The best way to resolve such issues is to use the built-in
Tempest helper function `get_unused_ip_addresses` defined
here [2].

This patch makes the necessary changes to the code in [1] and
to test_routers_rbac as well.

[0] http://logs.openstack.org/03/448603/7/check/gate-tempest-dsvm-patrole-admin-fast-ubuntu-xenial-nv/2bb077f/console.html
[1] https://review.openstack.org/#/c/448250/
[2] https://github.com/openstack/tempest/blob/master/tempest/common/utils/net_utils.py

Change-Id: I9bbb7cd77cde9287e398f506de58ff23e9e95496
Closes-Bug: #1675342
2 files changed