Discard hard drives that are less than minimum size

Add minimum acceptable size in settings.py(1GB). Update scan_disks to ignore
any hard drives that are less than the minimum size. This scenario occurs in
virtual machines with small block devices. Btrfs work with smaller than 1GB for
some raid configurations but not all. 1GB is some what arbitrary but solves the issue.
Merge pull request #38 from schakrava/issue#37_discard_small_disks

Issue#37 discard small disks