**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>