Implement copy_image_to_volume and copy_volume_to_image on nfs backends

Fixes: bug #1177288
Change-Id: I1c034de4acf13c52e399ba4623a0c8f2110fd695
Set default values for NFS/GlusterFS share_config files

By default, set:
 nfs_shares_config = /etc/cinder/nfs_shares
 glusterfs_shares_config = /etc/cinder/glusterfs_shares

This makes use of these drivers require one less manual
configuration step.

Also fix wording of help for glusterfs/nfs_mount_point_base
options.

Change-Id: I93bd3a8f08d4d5307f78e983d1080afae663dfe4
Merge "Set default values for NFS/GlusterFS share_config files"
Merge "Implement copy_image_to_volume and copy_volume_to_image on nfs backends"