Make the number of test iterations configurable via an environment variable.
Make the number of test iterations configurable via an environment variable.

This patch enables the user to set an environment variable specifying the number of
iterations to run all the tests. The configuration can be overriden via the command line.

Change-Id: I1da545201f1b59697344e62ae8edf5f9fb3cd92d
Reviewed-on: http://gerrit.cloudera.org:8080/188
Reviewed-by: Ishaan Joshi <d1d1e60202ec9f2503deb1b724986485a125d802@cloudera.com>
Tested-by: Internal Jenkins

michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala