Update secret docs and sample-functions

**What**
- Update the documentation about secret management to note the changed
file location
- Remove the documentation on secret rotation, because this will not
currently work
- Update apikey-secret and ApiKeyProtected-Secrets to read secret values
from both the old and the new locations

**Why**
- Recent updates to faas-swarm and faas-netes changed the mount location
of secrets.  These changes update the docs to reflect this change.

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

sample-functions/ApiKeyProtected-Secrets/handler.go

@@ -8,10 +8,21 @@ import (
 	"strings"
 )
 
+func getAPISecret(secretName string) (secretBytes []byte, err error) {
+	// read from the openfaas secrets folder
+	secretBytes, err = ioutil.ReadFile("/var/openfaas/secrets/" + secretName)
+	if err != nil {
+		// read from the original location for backwards compatibility with openfaas <= 0.8.2
+		secretBytes, err = ioutil.ReadFile("/run/secrets/" + secretName)
+	}
+
+	return secretBytes, err
+}
+
 func handle(body []byte) {
 	key := os.Getenv("Http_X_Api_Key")
 
-	secretBytes, err := ioutil.ReadFile("/run/secrets/secret_api_key")
+	secretBytes, err := getAPISecret("secret_api_key")
 	if err != nil {
 		log.Fatal(err)
 	}