[fuel-qa]Need to be able disable fuel-master auto-updates

[Aleksey Zvyagintsev]

Since after 9.1 release, 9.0 installation will fetch latest updates automatically [0]
we need to be able disable such auto-update function, to be able test plugins and other tools

We propose to do it with wait_for_external_config[1] hook, which also was implemented in
fuel-qa [2] long time ago

[0] https://github.com/openstack/fuel-main/blob/master/iso/bootstrap_admin_node.sh#L239
https://github.com/openstack/fuel-main/blob/master/iso/bootstrap_admin_node.sh#L27

[1] https://github.com/openstack/fuel-main/blob/master/iso/bootstrap_admin_node.sh#L396
[2] https://github.com/openstack/fuel-qa/blob/e624f14bcb1a4d5a3d1c0cb85c6e406c7bb8d0ab/fuelweb_test/models/environment.py#L433-L445

[Aleksey Zvyagintsev (azvyagintsev)]

Additional info :
fuel-release file will be always updated due:
https://github.com/openstack/fuel-main/blob/master/iso/bootstrap_admin_node.sh#L25
>>>
https://github.com/openstack/fuel-main/blob/master/iso/bootstrap_admin_node.sh#L232-L237