This commit fixes the naming inconsistency of Google Cloud DNS being
named `googlecloud` in the source and `gcloud` in the user interface.
Furthermore, improving my previous commit, I realized that the project ID
is stored in the service account file. We can save one environment variable
when using service account file (e.g. in automation applications).
When record already exists in gcloud we can't add a new record without removing the other one first. This is a simple fix that doesn't attempt to create multiple entries for the record but just removes the previous data.
fixes#218
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.
