Description of problem:
The 'samba4.blackbox.chgdcpass' test fails when Samba is built with MIT Kerberos.
This retries to establish a kerberos connections after removing a
ccache entry, to cope with a server being re-built while the client
still has a valid service ticket.
Please implement krb5_cc_remove_cred() for memory, file and keyring caches. The kcm cache already supports it.