This adds some text to the documentation about configuring the import
mechanism, including details about shared vs. local staging
directories. It also clarifies that *all* import methods require the
staging directory to be configured, as well as cleans up some
single-store-specific wording in this area.
Related to blueprint distributed-image-import
Change-Id: I726abe5d1104510e8da0e94f90f2b36d43b82cbe
d8a6309893