provider/azurerm: Fix panic if no creds supplied

Using EnvDefaultFunc with a default of empty string causes the
validation which would ordinarily be performed by `Required: true` in
the schema to not have any effect. Instead validate the configuration
used to produce the ARM client before attempting to use it during
provider configuration.

builtin/providers/azurerm/provider.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/Azure/azure-sdk-for-go/Godeps/_workspace/src/github.com/Azure/go-autorest/autorest"
+	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/helper/mutexkv"
 	"github.com/hashicorp/terraform/helper/schema"
 	"github.com/hashicorp/terraform/terraform"
@@ -75,6 +76,25 @@ type Config struct {
 	TenantID       string
 }
 
+func (c Config) validate() error {
+	var err *multierror.Error
+
+	if c.SubscriptionID == "" {
+		err = multierror.Append(err, fmt.Errorf("Subscription ID must be configured for the AzureRM provider"))
+	}
+	if c.ClientID == "" {
+		err = multierror.Append(err, fmt.Errorf("Client ID must be configured for the AzureRM provider"))
+	}
+	if c.ClientSecret == "" {
+		err = multierror.Append(err, fmt.Errorf("Client Secret must be configured for the AzureRM provider"))
+	}
+	if c.TenantID == "" {
+		err = multierror.Append(err, fmt.Errorf("Tenant ID must be configured for the AzureRM provider"))
+	}
+
+	return err.ErrorOrNil()
+}
+
 func providerConfigure(d *schema.ResourceData) (interface{}, error) {
 	config := Config{
 		SubscriptionID: d.Get("subscription_id").(string),
@@ -83,6 +103,10 @@ func providerConfigure(d *schema.ResourceData) (interface{}, error) {
 		TenantID:       d.Get("tenant_id").(string),
 	}
 
+	if err := config.validate(); err != nil {
+		return nil, err
+	}
+
 	client, err := config.getArmClient()
 	if err != nil {
 		return nil, err