grizzly: swift add fallocate_reserve option

If https://review.openstack.org/18290 is merged, a new option is added to swift: fallocate_reserve

    Some systems behave badly when they completely run out of space. To
    alleviate this problem, you can set the fallocate_reserve conf value
    to a number of bytes to "reserve" on each disk. When the disk free
    space falls at or below this amount, fallocate calls will fail, even
    if the underlying OS fallocate call would succeed. For example, a
    fallocate_reserve of 10737418240 (10G) would make all fallocate calls
    fail, even for zero-byte files, when the disk free space falls under
    10G.

    The default fallocate_reserve is 0, meaning "no reserve", and so the
    software behaves exactly as it always has unless you set this conf
    value to something non-zero.