Fix and test python scripts and kerberos

Fix and test python scripts and kerberos

This fixes up the python credentials interface in a number of areas,
with the aim of supporting '-k yes' as a command line option.  (This
enables the use of kerberos).

As such, I've had to change the get_credentials call to take a
loadparm context, so that the credentials can be initialised
correctly.

The test_kinit script has been modified to prove that this continues
to work, as well as to provide greater code coverage of the kerberos
paths.

Andrew Bartlett
(This used to be commit 727ef40c2b56910028ef3c1092b8eab1bfa6ce63)