Fully backwards compatible in terms of credential mechanisms
(environment variables, shared credentials file, EC2 metadata). If a
custom AWS IAM policy is in use it needs to be updated with permissions
for the route53:ListHostedZonesByName action.
Different DNS providers were handling credentials in different ways.
Some were reading credential environment variables in cli_handlers.go
and then passing them into the NewDNSProvider function, while others
were reading the environment variables within their NewDNSProvider
functions.
This change replaces each DNS challenge's NewDNSProvider function with
two new functions: (1) a NewDNSProvider function that takes no
parameters and uses the environment to read credentials, and (2) a
NewDNSProviderCredentials that takes credentials as parameters.
