nginx_proxy: Support integration with letsencrypt addon

[mraerino]

trafficstars

This turns out pretty handy when both addons are used alongside each other.

The only gotcha that people still have to care about is that you need to run the letsencrypt addon on port 80 once to get the initial cert material and then change its port and install nginx, because nginx won't start without a cert. Not sure if that could be documented better.

[mraerino]

@agners should we roll this into the same release as the IPv6 change?

[agners]

I rather prefer if we merge them separately with a separate version number, so we could easily roll-back (especially the IPv6 change).

Now in case the IPv6 change needs more discussions, maybe it is easier if we roll out this one first. You'd need to add changelog entries and bump the version in config.yaml here too, and then rebase the IPv6 change afterwards. Sorry for the inconvenience, the add-on core release process is not ideal currently :cry: