[tools] run rebalancer during 'election storm'
[tools] run rebalancer during 'election storm'

Added an integration test to run the rebalancer tool in an 'election
storm' environment.  The rebalancer should run with no issues
unless a tablet server is reported as unavailable.

I ran the test 250 times in each build configuration and didn't spot any
failures.

Change-Id: Ic98684dbe55049bbc411513faa0b6bbaef20f434
Reviewed-on: http://gerrit.cloudera.org:8080/11107
Tested-by: Kudu Jenkins
Reviewed-by: Will Berkeley <c74686ab413a71926fc1bc9d16ebfa30e619336a@gmail.com>

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