IMPALA-7124: delete keystore when formatting cluster
IMPALA-7124: delete keystore when formatting cluster

This avoids leftover keystore files from previous instances
of the cluster, e.g. testdata/cluster/cdh6/node-1/data/kms.keystore.
I checked on my local system and that is the only file outside of
the nn and dn subdirectories.

Change-Id: I9e6c1afa0df95b26446ccafa0c942e28ba848dae
Reviewed-on: http://gerrit.cloudera.org:8080/10613
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>
Reviewed-by: Philip Zeyliger <3ba3974e96044dcde818e6310ed799abe10671d4@cloudera.com>

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala