WIP: Vault End-Point Get/Head Update ** Needs Reviews before I can proceed**

CBDEUCE-89, CBDEUCE-111, CBDEUCE-112, CBDEUCE-113, CBDEUCE-114, CBDEUCE-115

DONE:
- Vault Head returns whether or not a Vault Exists
- Vault Get returns statistics on the vault
 - statistics are gathered from the drivers

Note: There are still several coverage issues in the drivers regarding how default values are returned.
deuce.drivers.cassandra.cassandrametadatadriver - lines 198-199
deuce.drivers.sqlite.sqlitemetadatadriver - lines 260-264
deuce.drivers.mongodb.mongodbmetadatadriver - lines 64,73, 82
deuce.drivers.swift.swiftstoragedriver - lines 86-87

Any suggestions on a good resolution welcome.

WRT res['internal'] - @TheSriram - whatever we thought relevant for the driver.

Essentially, right now I'm just putting out some basics - namely number of objects and total size; but also reserving an area (internal) for the driver in case there was anything specific to a driver that we wanted to add.

For instance, may be a driver stores data in a hierarchical structure, so we record the number of hierarchies there are and may be a max/min/mean of the number of objects at each level. It would only be relevant for that driver.

Now, the data might go beyond simple statistics. It's all a matter of what we may find relevant.

One aspect of this is that it could give us an area to add data without committing to always providing it for every driver. Then if we find it useful enough and relevant across multiple drivers we move it out as something official for all drivers.