tests: allow run-test.sh to auto-retry known-flaky tests
tests: allow run-test.sh to auto-retry known-flaky tests

This adds environment variables which can configure run-test.sh to
automatically retry known flaky tests a few times before failing.
The goal is to hook this up in the gerrit jenkins script so that
we first run determine-flaky-tests.py to find any tests which have
been failing recently on master, and then in gerrit builds allow
them to fail once or twice before actually giving a -1.

Hopefully this will save everyone a lot of trouble digging through
Jenkins on their gerrit builds, while still getting coverage on
those tests through the kudu-flaky-tests target.

Change-Id: I7d59f6d01bb0f8a5da367efa603aa6f6c5b733da
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3417
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Reviewed-by: Michael Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@cloudera.com>

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