Adding utility functions - IsNameOnly and WithDefaultTag

Signed-off-by: Nishant Totla <nishanttotla@gmail.com>

reference/normalize.go

@@ -0,0 +1,22 @@
+package reference
+
+var (
+	defaultTag = "latest"
+)
+
+// EnsureTagged adds the default tag "latest" to a reference if it only has
+// a repo name.
+func EnsureTagged(ref Named) NamedTagged {
+	namedTagged, ok := ref.(NamedTagged)
+	if !ok {
+		namedTagged, err := WithTag(ref, defaultTag)
+		if err != nil {
+			// Default tag must be valid, to create a NamedTagged
+			// type with non-validated input the WithTag function
+			// should be used instead
+			panic(err)
+		}
+		return namedTagged
+	}
+	return namedTagged
+}