DNSimpleProvider: fetch credentials from env

I also had to rename the `envAuth()` in the Cloudflare implementation
to avoid the "redeclared" error

    acme/dns_challenge_dnsimple.go:41: envAuth redeclared in this block
        previous declaration at acme/dns_challenge_cloudflare.go:154

acme/dns_challenge_cloudflare.go

@@ -20,7 +20,7 @@ type DNSProviderCloudFlare struct {
 // variables CLOUDFLARE_EMAIL and CLOUDFLARE_API_KEY.
 func NewDNSProviderCloudFlare(cloudflareEmail, cloudflareKey string) (*DNSProviderCloudFlare, error) {
 	if cloudflareEmail == "" || cloudflareKey == "" {
-		cloudflareEmail, cloudflareKey = envAuth()
+		cloudflareEmail, cloudflareKey = cloudflareEnvAuth()
 		if cloudflareEmail == "" || cloudflareKey == "" {
 			return nil, fmt.Errorf("CloudFlare credentials missing")
 		}
@@ -151,7 +151,7 @@ func sanitizeTTL(ttl int) int {
 	}
 }
 
-func envAuth() (email, apiKey string) {
+func cloudflareEnvAuth() (email, apiKey string) {
 	email = os.Getenv("CLOUDFLARE_EMAIL")
 	apiKey = os.Getenv("CLOUDFLARE_API_KEY")
 	if len(email) == 0 || len(apiKey) == 0 {

acme/dns_challenge_dnsimple.go

@@ -2,6 +2,7 @@ package acme
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/weppos/go-dnsimple/dnsimple"
 )
@@ -12,10 +13,14 @@ type DNSProviderDNSimple struct {
 }
 
 // NewDNSProviderDNSimple returns a DNSProviderDNSimple instance with a configured dnsimple client.
-// Authentication is either done using the passed credentials.
+// Authentication is either done using the passed credentials or - when empty - using the environment
+// variables DNSIMPLE_EMAIL and DNSIMPLE_API_KEY.
 func NewDNSProviderDNSimple(dnsimpleEmail, dnsimpleApiKey string) (*DNSProviderDNSimple, error) {
 	if dnsimpleEmail == "" || dnsimpleApiKey == "" {
-		return nil, fmt.Errorf("DNSimple credentials missing")
+		dnsimpleEmail, dnsimpleApiKey = dnsimpleEnvAuth()
+		if dnsimpleEmail == "" || dnsimpleApiKey == "" {
+			return nil, fmt.Errorf("DNSimple credentials missing")
+		}
 	}
 
 	c := &DNSProviderDNSimple{
@@ -34,3 +39,12 @@ func (c *DNSProviderDNSimple) Present(domain, token, keyAuth string) error {
 func (c *DNSProviderDNSimple) CleanUp(domain, token, keyAuth string) error {
 	return nil
 }
+
+func dnsimpleEnvAuth() (email, apiKey string) {
+	email = os.Getenv("DNSIMPLE_EMAIL")
+	apiKey = os.Getenv("DNSIMPLE_API_KEY")
+	if len(email) == 0 || len(apiKey) == 0 {
+		return "", ""
+	}
+	return
+}

acme/dns_challenge_dnsimple_test.go

@@ -1,17 +1,46 @@
 package acme
 
 import (
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
+var (
+	dnsimpleEmail  string
+	dnsimpleAPIKey string
+)
+
+func init() {
+	dnsimpleEmail = os.Getenv("DNSIMPLE_EMAIL")
+	dnsimpleAPIKey = os.Getenv("DNSIMPLE_API_KEY")
+}
+
+func restoreDNSimpleEnv() {
+	os.Setenv("DNSIMPLE_EMAIL", dnsimpleEmail)
+	os.Setenv("DNSIMPLE_API_KEY", dnsimpleAPIKey)
+}
+
 func TestNewDNSProviderDNSimpleValid(t *testing.T) {
+	os.Setenv("DNSIMPLE_EMAIL", "")
+	os.Setenv("DNSIMPLE_API_KEY", "")
 	_, err := NewDNSProviderDNSimple("example@example.com", "123")
 	assert.NoError(t, err)
+	restoreDNSimpleEnv()
+}
+func TestNewDNSProviderDNSimpleValidEnv(t *testing.T) {
+	os.Setenv("DNSIMPLE_EMAIL", "example@example.com")
+	os.Setenv("DNSIMPLE_API_KEY", "123")
+	_, err := NewDNSProviderDNSimple("", "")
+	assert.NoError(t, err)
+	restoreDNSimpleEnv()
 }
 
 func TestNewDNSProviderDNSimpleMissingCredErr(t *testing.T) {
+	os.Setenv("DNSIMPLE_EMAIL", "")
+	os.Setenv("DNSIMPLE_API_KEY", "")
 	_, err := NewDNSProviderDNSimple("", "")
 	assert.EqualError(t, err, "DNSimple credentials missing")
+	restoreDNSimpleEnv()
 }