Added retry for zookeeper_init per MESOS-1326.
Added retry for zookeeper_init per MESOS-1326.

Often during temporary DNS failovers or outages, we see slaves
aborting in zookeeper_init.

In many cases, the slave can restart within 10 seconds. Since
retrying zookeeper_init should be safe to do, this is an attempt
to minimize the number of unnecessary aborts in the slave.

Review: https://reviews.apache.org/r/21783

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