--kerberos fix

[Giampaolo Lauria]

Hi Jelmer,

Thanks for your quick answers.

While looking at your fix, I noticed that the function to handle the 
--kerberos option in class CredentialsOptionsDouble was not modified and 
is still as:

def _set_kerberos2(self, option, opt_str, arg, parser):
        if bool(arg) or arg.lower() == "yes":
            self.creds2.set_kerberos_state(MUST_USE_KERBEROS)
        else:
            self.creds2.set_kerberos_state(DONT_USE_KERBEROS)

Do we want a different logic for this type or was it just an oversight?

In general, could someone explain to me the reason for having two 
different classes for these options. It seems like the implementation is 
the same for both classes w/ the exception of --ipaddress and the suffix 2 
on pretty much all of its fcns and variables. Currently, one class derives 
from the other, yet most of the code is duplicated.

I can make the necessary changes once I get your permission and 
clarification as to what each class is trying to represent.

Thanks,
Giampaolo