ACCUMULO-3197 Remove deprecated Instance.getConfiguration()

  This patch removes the deprecated setter/getter which had poorly-defined
  behavior in previous versions, and was prone to incorrect use. It replaces it
  with a less risky getter on ZooKeeperInstance only, and refactors utility code
  with better names and documentation to help clarify correct usage and
  limitations. This mitigates some of the problems identified in ACCUMULO-3199,
  by removing the problematic deprecated public API methods while that issue
  awaits completion.