Make Acmeproxy ACME Account configurable

[fhofherr]

Currently acmeproxy uses arbitrary data to create an ACME account for itself:

* A random UUID is used as the account ID
* The domain is hard-coded to `www.example.com`
* The empty string is used as email for the ACME account

In order to be useful, the behavior has to be changed to the following:

• Domain and Email have to be configurable. Either by using a configuration file / environment variables or by an admin interface.
• If acmeproxy did not create an ACME account yet, it may continue to use a random UUID to create a new account. As soon as it created the account it must reuse it and thus check if there is already an UUID.

Note: the UUID is used to identify the account within acmeproxy only. It is of no use for the ACME CA.