Add helper functions for getting local disk

Following functions are added in order to be used by validate_bdm
and resource_tracker code.
get_bdm_ephemeral_disk_size: return sum of eph disk size of bdms
get_bdm_swap_list: return a swap disk list of bdms
get_bdm_local_disk_num: return local disk number (include eph and swap)

Change-Id: I215970bea72b502042472aa310fe64ffc4304b25
Merge "Add helper functions for getting local disk"