Gluster: add the Shared File storage type.

The shared file and gluster disk templates should not report their disk
space information like file does, because they do not behave the same.

If a cluster pulls from the same, shared source of storage then it is
not correct, or useful, to report per-node disk availability, as the
information is not node-specific.

This change introduces the Shared File storage type for which no
per-node measuring of disk resources is made.
Gluster: announce in NEWS

Add the relevant line to NEWS