Merging two mkfs commands.

nova.disk.api.mkfs will use nova.utils.mkfs,
unless an arbitrary mkfs command has been provided by the user.
Preserving the default filesystem to be used for each os_type.

Also, adding run_as_root parameter to utils.mkfs, to allow it
handle block devices.

Change-Id: I3ca21e0ee80caa7e1152ebae56d73ed3c14844c0
Merge "Merging two mkfs commands."