Implement a fake SDL database backend

Implement a fake SDL database backend what can be used only for
local testing purposes, when SDL client does not need real DBAAS
database services. An SDL client can activate fake SDL database
backend usage when SDL instance is initiated with optional
'fake_db_backend' parameter:
  sdl_instance = SyncStorage(fake_db_backend='dict')
Value 'dict' is only supported.

Change-Id: I2ee7474175d0923617e026c306ec684b117dab60
Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>