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.