Add attribute filesystem_store_metadata_file

Attribute name: filesystem_store_metadata_file
File name: /etc/glance/glance-api.conf
Section: default

Description: A path to a JSON file that contains
 metadata describing the storage system. When
show_multiple_locations is True the information
in this file will be returned with any location
that is contained in this store.

Change-Id: I3b2a6b9da99ca061012c8b808c3821072ec63f9b
Closes-bug: #1348015