Parameterized search paths for default keytab, ccache

[Sam Hartman]

I think for the keytab use cases you need file existence for read
operations.

I actually think for the keytab use cases you'd probably be OK with
first existing entry or last entry if none of the previous entries
exist.

Rationale for why I think file existence is necessary for keytab:
I want my acceptor to use per-user acceptor keytab if it exists else
/etc/krb5.keytab.
for that I don't want to have to add an extra level of directory that
can not exist for the test to work.

--Sam