Use common identifiers package with less restrictive regex

Regular expressions in the namespaces package are redundant with the
pre-existing validator in the "identifiers" package; replace this custom
usage with the validator in "identifiers"

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

events/exchange/exchange.go

@@ -225,7 +225,7 @@ func validateTopic(topic string) error {
 }
 
 func validateEnvelope(envelope *events.Envelope) error {
-	if err := namespaces.Validate(envelope.Namespace); err != nil {
+	if err := identifiers.Validate(envelope.Namespace); err != nil {
 		return errors.Wrapf(err, "event envelope has invalid namespace")
 	}