Allow arbitrary paths

[aureq]

It would be desirable to have arbitrary path for the following items:

• the configuration file
• the lambda package
• the location under which the certificates are stored

[aureq]

So, the idea is as follows:

• The Lambda function is called with a bucket and a region parameter to fetch the configuration. This will become the very default to store/access keys and certificates.
• 3 new global configuration directive (region, bucket, path) will be added to store all the keys and certificates.
• Per domain settings will allow the default and global values to be overridden.

This implementation will require a new Lambda function parameter where letslambda.yml location and name can be specified. If missing, it will default to /letslambda.yml as it currently is.