Remove Yarn from minicluster by default.
Remove Yarn from minicluster by default.

Turns out that we start Yarn as part of the minicluster, but we never use it.
(HiveServer2 is configured to run MR jobs "locally" in process.) Likely, this
Yarn integration is a vestige of Yarn/Llama integration.  We can save memory by
not starting it by default.

There are some less-common tooks like tests/comparison/cluster.py which use
Yarn (and Hadoop Streaming). In deference to those tools, I've left a mechanism
to start Yarn rather than excising it altogether. After running
buildall the regular way, add Yarn to the cluster by running:
  testdata/cluster/admin -y start_cluster

I tested by running core tests. I did not test the kerberized minicluster.

Change-Id: I5504cc40b89e3c6d53fac0b7aa4b395fa63e8d79

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