Make NFS share selection more intelligent.

Make the NFS share selection more intelligent by using a used_ratio
and oversub_ratio to determine the valid target.  The used_ratio
is a hard-cap of maximum "actual" used space, oversub_ratio allows
for oversubscription of the share based upon the apparant-allocated
space for sparse files.

Removed Options: nfs_disk_util

New options added:
* nfs_used_ratio:
    Float representation of the maximum allowed usage (e.g. 0.95
    calculated by available_space / total_space) before the share
    is no longer concidered a valid target.  Default: 0.95

* nfs_oversub_ratio:
    Float representation of the oversubscriotion ratio allowed when
    utilizing sparse files (determined via "du").  This gets
    multiplied against the total_capacity before comparing the
    apparant space used by the backing volume files.  Default
    ratio is 1.0 (meaning reserved/allocated space cannot exceed
    total space available on the mount).

DocImpact: New Configuration Options / Removal of Config Option

bp nfs-share-selection-logic-improvement

Change-Id: I3572e106eb4c794b08bf6f499e2da2d494c4072d

Conflicts:
	cinder/volume/drivers/nfs.py
Declare compat object CONF which is a copy of FLAGS

Instead of backporting the FLAGS -> CONF patchset we just
go with a compatibly patch