Bug 1002 - REST POST transaction is not commited

fix REST POST create workflow (create data only), fix according to specification
  http://tools.ietf.org/html/draft-bierman-netconf-restconf-03#section-2.4

I'm missing implementation for ietf-restconf:errors, so I can't do more for now
  http://tools.ietf.org/html/draft-bierman-netconf-restconf-03#page-48

* fix a check for an existing node in DataStore
* add warn log and a correct ResponseException
* fix tests for BrokerFacade

Change-Id: I68c95e831f42cae4df0608a40c585251a631693f
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>
Merge "Bug 1002 - REST POST transaction is not commited"