'prefixlen' argument is absent for 'subnet' method in 'change_public_vip' test

[Tatyana Kuterina]

Detailed bug description:
'prefixlen' argument is absent for 'subnet' method in 'change_public_vip' test
File: fuel-qa/fuelweb_test/tests/test_manual_vip_allocation.py
string: 76

Found on CI:
https://product-ci.infra.mirantis.net/job/9.x.acceptance.ubuntu.change_vip_manually/2/console

Test: Deploy cluster with public vip manually set
Test Group: change_public_vip

Steps to reproduce:
        1. Create cluster
        2. Add 1 node with controller role
        3. Add 1 node with compute role and 1 cinder node
        4. Change public vip value to ip address from public range
Expected results:
  Test is passed
Actual result:
  Test is failed with error:
      TypeError: subnet() takes at least 2 arguments (1 given)

Description of the environment:
 9.1 snapshot #93

FUEL_QA_COMMIT=870dbbe7a6c9186cc3512f671d9292a2123aa35f

https://drive.google.com/a/mirantis.com/file/d/0Bz15vbpS5ZPNblFaa3RQQTJ4SmM/view?usp=sharing

[Alexandr Kostrikov (akostrikov-mirantis)]

@Andrey, please, explain why we use [0][5]. Does it mean 5th IP in first subnet? What is a preferred prefix?
        ip_to_set = str(
            self.env.d_env.get_network(name='public').ip.subnet()[0][5])

[Nastya Urlapova (aurlapova)]

Should be fixed by https://review.openstack.org/#/c/380076/

[Dmitry Belyaninov (dbelyaninov)]

Passed:
https://product-ci.infra.mirantis.net/job/9.x.acceptance.ubuntu.change_vip_manually/14/