Add optional programmatic configuration of the DNS provider

[remilapeyre]

In remilapeyre/vault-acme#19 we would like to be able to use instantiate the same provider multiple times with a different configuration. We could use os.Setenv() to change the environment variables before doing this but it is cumbersome and would require to have a Mutex to avoid race conditions.

This patch add a new config parameters where the values for the configuration is first looked into before looking in the environment variables. Callers that only want to use the environment variables can set this parameter to nil and the old behaviour of looking in the environment variables will be kept.

[ldez]

Same topic as #605

The env vars are not really configurable in the way that your thinking: some providers are more complex to handle configuration than env vars inside lego.

Your approach can works for some providers but it's not a valid global approach.

[ldez]

I would propose you to open an issue to explain your use case and be able to discuss around this topic.

As I explain in my previous message, this PR, even it the approach is interesting, can not be merge because it's not a valid global approach.

Thanks anyway.