Force the use of a separte Kerberos cache per thread

When using a single Kerberos cache that is shared among threads,
Koji logins start failing because the cache gets corrupt. This uses
the Linux kernel keyring to store a Kerberos cache per MBS thread.

See https://web.mit.edu/kerberos/krb5-1.12/doc/basic/ccache_def.html

conf/config.py

@@ -77,7 +77,6 @@ class BaseConfiguration(object):
     # Settings for Kerberos
     KRB_KEYTAB = None
     KRB_PRINCIPAL = None
-    KRB_CCACHE = None
 
     # AMQ prefixed variables are required only while using 'amq' as messaging backend
     # Addresses to listen to