fuel-devops: Support master node installation as node extension for Fuel for OpenStack

There are 2 places in our code where we do installation of admin node: - fuel-qa/fuelweb_test/models/environment.py::EnvironmentModel::setup_environment - fuel-devops/devops/shell.py::Shell::do_admin_setup We want to combine them and make one generic way to setup admin and use it in both places.