Allow any credential to be used for machine credentials

Don't restrict machine credentials to be "nfs/<machine.name>".
Use any usable credentials contained in the keytab file.
[We actually attempt to use the first entry found for each
realm, not every entry, in the keytab.]

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>