Support pulling configuration via consul/etcd

[dmolik]

Description

given a consul/etcd endpoint given as a command line parameter, gitea should be able to pull further configuration via an api. Furthermore, gitea could register itself and monitoring checks.

[bkcsoft]

If we supported more env-vars (using e.g. https://github.com/kelseyhightower/envconfig) one could use https://github.com/mattn/etcdenv for this. As for SIGHUPing gitea when config changes, that's up to etcdenv (or similar tooling) IMO

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.