[9.x acceptance] an invalid definition of controller node

[Sergey Novikov]

Detailed bug description:

The issue was found by https://product-ci.infra.mirantis.net/job/9.x.acceptance.ubuntu.iac_git_download_settings/9/testReport/(root)/iac_git_download_settings/

Root cause of the failure is an invalid definition of controller node.
https://github.com/openstack/fuel-qa/blob/stable/mitaka/fuelweb_test/tests/test_iac_git_extension/test_iac_git_extension.py#L148
There is we assume that first nodes what was defined by nailgun are controller nodes - it's wrong.

So, we need to predifine controller node by https://github.com/openstack/fuel-qa/blob/stable/mitaka/fuelweb_test/models/fuel_web_client.py#L1307-L1317