Use mirrors snapshots

Use mirrors snapshots

Add logic to fetch information about last suitable snapshot which can be
used to download ubuntu packages. This is for example use to build
community ISO [1]. This should help in case of snapshot switch [2].

[1]
https://github.com/fuel-infra/jenkins-jobs/blob/master/servers/fuel-ci/8.0/builders/community.all.sh#L36-L40
[2] https://bugs.launchpad.net/fuel/+bug/1493390/comments/9

Change-Id: Id5a8d41278561473ea4844631bfefa0049e64dbc
Signed-off-by: Michal Skalski <mskalski@mirantis.com>