Introduces a new command line switch `--must-staple` to `run` and `renew`.
Using this switch will add the must staple TLS extension to the CSR generated by lego and thus also to the generated certificate.
This does not work with user specified CSRs!
Fixes#270
Added a slice of NS to be used when retrying queries. Also used with FindZoneByFqdn()
Adjusted 2 error messages given to better differentiate the returned error string
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.
