Added command `knife opc user password`.

Command either updates the password for a user or, if --enable_external_auth is passed instead of a password, it will enable ldap for that user.
`<>` isn't a common thing to see in a command synopsis.  Many unix tools punt on a situation like this and split the usage to multiple lines.  For instance, BSD cut:

```
usage: cut -b list [-n] [file ...]
       cut -c list [file ...]
       cut -f list [-s] [-d delim] [file ...]
```
Which in our case would be:
```
usage:  knife opc user password USERNAME PASSWORD
        knife opc user password USERNAME --enable_external_auth
```
Unfortunately, mixlib-cli doesn't give us the ability to do that.  I'd prefer if we did:

    knife opc user password USERNAME [PASSWORD | --enable-external-auth]