fix metal provider flag and env loading, and return error if provider not defined

internal/metal/config.go

@@ -1,6 +1,9 @@
 package metal
 
 import (
+	"github.com/spf13/pflag"
+	"github.com/spf13/viper"
+
 	"go.equinixmetal.net/infra9-metal-bridge/internal/metal/providers/emapi"
 	"go.equinixmetal.net/infra9-metal-bridge/internal/metal/providers/emgql"
 )
@@ -13,3 +16,9 @@ type Config struct {
 	// EMAPI sets the provider to Equinix Metal API.
 	EMAPI emapi.Config
 }
+
+// MustViperFlags registers command flags along with the viper bindings.
+func MustViperFlags(v *viper.Viper, flags *pflag.FlagSet) {
+	emgql.MustViperFlags(v, flags)
+	emapi.MustViperFlags(v, flags)
+}