GlusterFS volume layout pool filtering via user.*

In GlusterFS volume layout, currently a volume's mebership in the volume pool is decided based on pattern-matching on its name.

An additional mechanism is needed whereby the membership is decided based on some user.* option (tentatively user.manila-pool). Indication of volume size should also be possible through an user.* option (tentatively user.manila-size). These will provide a more flexible scheme to update pool membership and to indicate an up-to-date size (as volume options can be changed any time, while volume rename is not supported).