Add teststep.Config interface so that logic that examines the content…

…s of TestStep.Config or TestStep.Directory can be encapsulated in teststep.config struct (#150)

internal/teststep/config.go

@@ -0,0 +1,54 @@
+package teststep
+
+import (
+	"fmt"
+	"strings"
+)
+
+type Config interface {
+	HasConfiguration() bool
+}
+
+type configuration struct {
+	raw       string
+	directory string
+	file      string
+}
+
+func Configuration(raw, directory string) (configuration, error) {
+	var populatedConfig []string
+
+	if raw != "" {
+		populatedConfig = append(populatedConfig, fmt.Sprintf("%q", "raw"))
+	}
+
+	if directory != "" {
+		populatedConfig = append(populatedConfig, fmt.Sprintf("%q", "directory"))
+	}
+
+	if len(populatedConfig) > 1 {
+		return configuration{}, fmt.Errorf(
+			"both %s are populated, only one configuration option is allowed",
+			strings.Join(populatedConfig, " and "),
+		)
+	}
+
+	return configuration{
+		raw:       raw,
+		directory: directory,
+	}, nil
+}
+
+func (c configuration) HasConfiguration() bool {
+	if c.raw != "" {
+		return true
+	}
+
+	// TODO: do we need to read each file in directory and check that it's not empty.
+	// TODO: do we also need to verify that it's valid TF config
+	if c.directory != "" {
+		return true
+	}
+
+	return false
+}