File provider

[wovalle]

Angelos supports dynamic providers such as Docker and traefik but I think that many people would benefit of having a more static provider such as a file.

Open questions:

• How the schema would look like
• Not a question but it should implement IMetadataProvider
• Should the file be only used as a way to specify the hosts or it should provide an api to define other params (such as the values we pass as env vars?)