XD-1851 Introduce cache to ZooKeeperContainerRepository

Introduced PathChildrenCache to ZooKeeperContainerRepository
to address the concerns indicated in
https://github.com/spring-projects/spring-xd/pull/936

Container cache no longer needs to be a member

Since the container cache is no longer externally accessible
due to ZooKeeperContainerRepository, this was moved
to be a local variable within the leader election method.
XD-1775 & XD-1781 HdfsJdbc, HdfsMongo Accept Tests

* Address comments in PR review https://github.com/spring-projects/spring-xd/pull/914/
XD-1814 Acceptance Tests for Labels and Taps

* Includes Transform fixture as well.
* Improvements to Result File Validator.
** The file validator now checks to see if the file is present and loops
   for a period of time with one second pauses if not present.
   Before we always waited pausetime *2000 (millis).
** Runtimes were about 17 min with the latest checks.  Now they are
   running around 12 min.
* testStreamTapTwoProcessors Uses labels vs. module index.
* Delimiter now spelled correctly
* Thread.currentThread().interrupt() is now in the catch block for the thread.sleep
* Removed code for index that was no longer relevant.
Fix ContainerRepositoryTest

- Add explicit check to verify if the saved containers are found
within a specified time (15s).