CDH-18751: Impala should use a unique Kerberos credentials cache location

Kerberos, by default, uses a location for its credentials cache (where
tickets are saved to avoid having to re-authenticate on each connection)
that is unique only to the user. This means that if several programs
running as the same user try and authenticate concurrently, they may
race to access the cache, and an error might occur.

This patch causes Impala's demons to use a path that is unique to
process name and process id (e.g. /tmp/krb5_impalad_4324). The new flag
--krb_credential_cache can be used to set the location explicitly.

Any Kerberos connections initiated from Java will still use the system
default cache. The catalog service is the only such service. It's
important that the cache locations for Java and C++ are different (to
avoid the same bug being exposed in a single process, which is actually
its most likely cause). The Java location can be changed by setting the
KRB5CCNAME environment variable.

Change-Id: I4d975a3a7cf5ff6ee4c107833f5292115cd33b03
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2308
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins
Fix two unused variable warnings

Change-Id: Ib970d8300ba289455fcbc6843001c8ec6844e009
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2340
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins