zk updater: handle retry failures

On NodeExists and NoNode failures, assume that we're out of sync and
resync, with exponential backoff.
do not run zk updater tests in parallel

Avoid spawning a lot of sub processes, which makes
travis sad.
cli: less verbose duration formatting
Merge pull request #40 from spotify/dano/cli-less-verbose-duration-format

cli: less verbose duration formatting
Merge pull request #39 from spotify/dano/zk-updater-handle-retry-conflicts

zk updater: handle retry failures
Merge pull request #38 from spotify/dano/zk-updater-test-no-parallel

do not run zk updater tests in parallel
sys test: teardown: do not assume setup succeeded

If setup fails before zk was assigned, teardown would NPE.
move docker probing to system test

Fail faster when docker is down/unreachable.
sys test: remove unused executor
disable curator failover test

This test is only in there to build confidence that a three node
zookeeper can recover from losing a node.

It takes long time to run serially and spawns a lot of subprocesses
concurrently when run in parallel, so it likely causes problems on
travis.
travis: single-thread parallelized tests
travis: longer zk curator timeouts
travis: deploy fewer hosts in cluster test
travis: do not run tests in parallel

Attempt to use less resources and avoid spurious failures.
travis: log WARN at least
test: exit if zk processes exit unexpectedly

Attempt to fail fast if e.g. the OOM killer in travis kills the zk
processes.
travis: only build using oracle jdk

Building with openjdk does not give us much. Since java 7, openjdk and
oracle jdk are similar enough that if something runs on the oracle jdk,
it will very likely work on openjdk as well. Also, we do not use
openjdk ourselves.

As such, the build agent that gets consumed by the openjdk can be better
used building other PR's.
temporary ports: do not release file lock twice 

Also change unlock failure logging to warn level.
test: restore zk updater test run parallelism
zk updater: less verbose failure handling

Do not log stack traces for expected zk failures.