[debops.elastic_co] Proxy support in Stretch

Right now it's not possible to add APT repository GPG key when machine doesn't have direct access to the internet. Ansible apt_key module uses apt-key command which uses gpg2 in Stretch. gpg2 uses dirmngr and you can set proxy configuration in dirmngr.conf but it's not possible with apt-key (it creates temporary configuration).

This issue applies not only to this role but to many other like nodejs or nginx. Simple grep -r keyserver: | wc -l in my not so up to date configuration shows 26 occurrences.

There is a numer of potential solutions:

1. Don't use apt_key and do all keyring manipulation manually
2. Don't use id in apt_key, use data and place whole key in defaults
3. Add an variable which would allow to use data instead of id. id would be used by default (so the key would be fetched form key server) but it will be possible switch to data if needed.

Start by inspecting the debops.elastic_co role and searching the repository's keyserver occurrences, including the nodejs and nginx roles. Compare the proposed apt_key, manual keyring, and data-based approaches; done requires a decided proxy-compatible way to add APT repository keys across the affected roles.