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.

I would say the model should behave consistently. If we define all models to internally work one way that's fine as long as it's consistent; whether or not the internal model maps directly to external end-points is an open ended question. Do we want to do that?

I don't know; would provide consistency between end-points and the models, but sometimes it's better to make the internal models more explicit and self-documenting than necessarily mapping to the external API that we don't necessarily have as much control about.

So, I'd probably not implement the model as "HEAD" or "GET", but "vault_exists()" and "vault_get_statistics()", which is what I think we are doing.

Or am I misunderstanding the question? (@TheSriram)