Storage planning info

Moved info from ref-arch to planning guide
- renamed file to 0100-storage-for images
  & subject to "Storage for Images"
- restructured information to read like planning issues
- removed list of Ceph limitations fixed in MOSt but not vanilla
  OpenStack -- they are moved to the ref-arch Ceph section
- now includes ephemeral as well as persistent storage;
  not a lot to say about ephemeral

terminology:  add ephemeral storage, lvm, persistent storage

user-guide:
- fix xref to planning material in configure/3500-cinder-glance-backend
- add tag to NTP section in initialize/400-pxe
- add config/settings/qcow to show the choice between qcow and raw
- added note to Settings/storage about selecting ephemeral storage

Closes-Bug: 1375823

Change-Id: I264e5c5fedac696e92d027b5e500b21dd74fb47e
Merge "Storage planning info"