Copter: Ch7/8 option to retract mount

-Define AUX_SWITCH_RETRACT_MOUNT in defines.h
-Added 27=Retract Mount to Parameters.pde CH7 and CH8
-added int on CH7/8opt AUX_SWITCH_RETRACT_MOUNT
-Added Case #if Mount == ENABLE to set mount mode
made set_mode public in AP_Mount.h lib
Mount: set_mode method made public