[fuel-qa] Unable to pass EXTRA_DEB_REPOS to tests without explicitly defining MIRROR_UBUNTU

[Sergey Kulanov]

We are not able to add EXTRA_DEB_REPOS in astute.yaml [1] without explicitly defining MIRROR_UBUNTU [2]

Probably we need to change logic:
- replace_ubuntu_repos should do replacement only
- and/or use add_ubuntu_extra_mirrors when EXTRA_DEB_REPOS defined separately.

The above is true for EXTRA_RPM_REPOS

[1]. https://github.com/openstack/fuel-qa/blob/master/fuelweb_test/helpers/replace_repos.py#L66
[2]. https://github.com/openstack/fuel-qa/blob/master/fuelweb_test/helpers/fuel_actions.py#L273