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.
Use GCE_PROJECT to designate your GCE project.
Authentication is automatically picked up from gcloud credentials if
running locally and from GCE metadata if run within Google Cloud.
Requires at least permission scope
"https://www.googleapis.com/auth/ndev.clouddns.readwrite"
