Added fallocate_reserve option

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 5368709120 (5G) would make all fallocate calls
fail, even for zero-byte files, when the disk free space falls under
5G.

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.

Also fixed ring builder's search_devs doc bugs.

Related: To get rsync to do the same, see
https://github.com/rackspace/cloudfiles-rsync
Specifically, see this patch:
https://github.com/rackspace/cloudfiles-rsync/blob/master/debian/patches/limit-fs-fullness.diff

DocImpact

Change-Id: I8db176ae0ca5b41c9bcfeb7cb8abb31c2e614527
Merge "Added fallocate_reserve option"