RAID 50/60 Fixes: Pick better `pdperarray` defaults

[caladd]

The code, at least in controller_megacli.py, currently defaults number of disks to 5 and 6, which should be changed to 3 (RAID50) and 5 (RAID60).

New logic should be added after the number of arrays is calculated, to divide up the slots into equal sized groups, and calculate the final pdperarray number.

Creating a RAID50 with 8 disks, should automatically choose to create the array with two groups of disks with 4 disks per array, when the pdperarray option is not specified. Similarly, a RAID60 with 8 disks should also create two groups of disks with 4 disks per array.