added fab switch command, for switching branches.
added fab switch command, for switching branches.

Usage: fab prod switch:realease_6

yaniv14/OpenCommunity,hasadna/OpenCommunity,yaniv14/OpenCommunity,yaniv14/OpenCommunity,hasadna/OpenCommunity,nonZero/OpenCommunity,yaniv14/OpenCommunity,nonZero/OpenCommunity,nonZero/OpenCommunity,hasadna/OpenCommunity,hasadna/OpenCommunity,nonZero/OpenCommunity