cannot add repository with deb-line and gpg public key

Currently maas-deployer and cloud-init do not support http_proxy[1][2], apt_sources with "source: ppa:maas/stable"[3] will fail if VM does not have direct internet connectivity.

It would be nice if maas-deployer supported more general cloud-init syntax[4] so that we could add PPAs without http_proxy, but apt_proxy only. It won't be a root fix for http_proxy problem, but it works as a workaround.

[1] https://bugs.launchpad.net/maas-deployer/+bug/1491556
[2] https://bugs.launchpad.net/cloud-init/+bug/1089405

[3] maas_deployer/vmaas/templates/cloud-init.cfg
{% if apt_sources -%}
apt_sources:
{% for source in apt_sources -%}
  - source: "{{ source }}"
{% endfor %}
{%- endif %}

[4]
apt_sources:
  - source: deb http://ppa.launchpad.net/maas/stable/ubuntu trusty main
    filename: maas-stable-trusty.list
    key: |
      -----BEGIN PGP PUBLIC KEY BLOCK-----
      Version: SKS 1.1.5
      Comment: Hostname: keyserver.ubuntu.com

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      =QeWQ
      -----END PGP PUBLIC KEY BLOCK-----