look in the file bed-os/targets/TARGET_STM/TARGET_STM32L0/TARGET_STM32L072xZ/TARGET_DISCO_L072CZ_LRWAN1/PeripheralPins.c for PinMap_I2C_SDA and PinMap_I2C_SCL.
You will see all the options you have available and if pin already in use for other purpose.