Update battery monitoring code for ArduPlane

Fixes compatibility for APM2.  Also a significant update to the battery monitoring code:  We previously had monitoring modes for individual cell voltages for 3 and 4 cell lipos.  These have been removed as they were never really supported (the cell voltages were computed but were not reported or recorded anywhere).  Also, some clean-up/prep work was done for supporting monitoring 2 separate battery packs.  The CLI battery and current monitoring tests were consolidated into 1 test.