Add multifilesystem store to support NFS servers as backend

Currently to configure multiple nfs server as a backend
using filesystem store, you cannot mount all disks to a single directory.
Filesystem store allows administrator to configure only single directory with
filesystem_store_datadir parameter in the glance-api.conf.

With multifilesystem store, administrators can configure multiple directories
to store the glance image. Each directory can be coupled with its priority.

Current Format for filesystem_store_datadir in glance-api.conf:
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filesystem_store_datadir=</path/to/store>
filesystem_store_datadir = /var/glance/store1

Changed Format for filesystem_store_datadir in glance-api.conf:
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filesystem_store_datadir=</path/to/store>:<priority>,.....
filesystem_store_datadir = /var/glance/store1:100,/var/glance/store

Note:
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1. Store with priority 200 has precedence over store with priority 100
2. If no priority is specified, default priority '0' is associated with it.
3. If two filesystem stores have same priority store with maximum free space
will be chosen to store the image.

blueprint: glance-multifilesystem-store
DocImpact: Miltifilesystem support can be used by specifying multiple
directory paths to filesystem_store_datadir param in glance-api.conf as
discussed above.