bootstrap_admin_node: Fix connectivity check.

bootstrap_admin_node: Fix connectivity check.

After Fuel Master builds the bootstrap image at first boot,
internet connectivity is tested using the URL for the CentOS
updates repository.

bootstrap_admin_node.sh script parses the output of `yum repolist`,
looking for entries like 'Repo-baseurl: http://...'.

However, in Fuel 9.0, <updates> points to a mirrorlist file (a text
file containing a list of possible mirror URLs) instead of being
a single, direct link to the repository.

Also, note that in Fuel9, <updates> and <security> repos are disabled
by default.

For now, we only test that we can access the mirrorlist file,
while the check should be performed against the mirror(s).

Old (Fuel 8) repo entry format:
Repo-baseurl: http://mirror.fuel-infra.org/.../updates/x86_64/

New (Fuel 9) mirrorlist format:
Repo-mirrors: http://mirror.fuel-infra.org/.../mos-mirrors-updates.txt

Backported from upstream change [1], which should fix bug [2].
This change will get merged upstream in Fuel 10.

[1] https://review.openstack.org/#/c/335094/
[2] https://bugs.launchpad.net/fuel/+bug/1596981

Change-Id: Ibec397af2e1d925a011907f14e1d655741e280fb
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>