Bug 1226776

Description of problem:

We need an interface, exposed through cli and gfapi, that allows the requestor to get information about the capabilities of a volume. What a capability is not defined here exactly, however, we mean high-level (user centric) properties (beyond the technical parameters that can be interacted with through "volume info / set"). The information given should be something like a scalar valued dictionary, self-contained and human friendly (ie. a user should be able to make sense of it without anything but basic familiarity of GlusterFS). 

The attainment of a capability on behalf of a volume depends on three factors:
- GlusterFS version (support of a feature at source level)
- GlusterFS build/deployment (support of a feature in the binary)
- particularities of the volume.

So for example take snapshot support: AFAIK only thin provisioned lv volumes support snapshot, therefore snapshot support is a per-volume property.

Currently we have in mind as possible capabilites (shown here just to give a taste of the idea, not meaning we'd need these in this exact form - also note that of some of these refer to features that are still just RFEs):

- volume snapshot
- volume snapshot clone
- directory snapshot
- directory snapshot clone
- cert based auth
- cert based auth for directories
- thin provisioned

Upon implementing this RFE a broad scan of capability candidates should be executed.