A user can specify a password longer then 8 characters.
MAN pages states:
```
cisco-authentication secret
Enables Cisco style authentication on NHRP packets. This embeds
the secret plaintext password to the outgoing NHRP packets.
Incoming NHRP packets on this interface are discarded unless the
secret password is present. Maximum length of the secret is 8 characters.
```
We should print a warning for the user and then truncate the password