image: Add hook to customize metrics server URLs

[dbnicholson]

Allow configuring the metrics server URLs so that 3rd parties deploy the Endless metrics stack they can build images to use them out of the box.

https://phabricator.endlessm.com/T35420

[dbnicholson]

I'm marking this as a draft since we may not want to bother with the eos-phone-home dropin if endlessm/eos-phone-home#18 is merged.

[cassidyjames]

endlessm/eos-phone-home#18 indeed seems cleaner, so I think I prefer that unless there's a good reason to do it this way.

Edit: ah, but we may want to merge this first since it adds the metrics_server_url and
activation_server_url keys to the config, then we can update the hook itself to write the config file, or…?

[dbnicholson]

I hate the idea of editing an INI file with sed and echo so I had a brief foray into "proper" editing using configparser. But it turns out that it's write method is fairly barebones, so I scrapped it. This should be good enough knowing that both files only have a single global section.