Specify supported architectures for Ubuntu backports repository

Specify supported architectures for Ubuntu backports repository

Currently Trusty (Ubuntu 14.04 LTS) and Xenial (Ubuntu 16.04 LTS) doesn't
have support for arch=ARM64 in backports repository:
- http://archive.ubuntu.com/ubuntu/dists/xenial-backports/
- http://archive.ubuntu.com/ubuntu/dists/trusty-backports/

During the Yardstick image building process, the repository is added to the
APT repository list. In order to avoid errors during the repository retrival,
the available architectures must be specified [1].

[1]https://github.com/opnfv/yardstick/blob/stable/euphrates/ansible/roles/add_custom_repos/templates/sources.list.j2#L4

JIRA: YARDSTICK-874

Change-Id: I7774189fbdd7d74ff5df29526ba0da2693b3b53a
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>