CMakeLists.txt can now switch between db backends

Added ability to select database backends like leveldb and berkeleydb via
WEBOS_DB8_BACKEND cmake variable. All specific db wrapper configuration moved to
src/db-luna/CMakeLists.txt

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <msditanov@luxoft.com>

Change-Id: I893772b51274ea41a1b6497909b7c7233915d4c2