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