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.

Looking at this lines...Vault.get(...) retrieves the vault model or returns None; it's just a static access function; not returning stats or anything else. So I'm not sure the applicability of the original comment here.

The original functionality was the the Get on the Vault Controller returned the existence instead of the statistics; while the Head on the Vault Controller returned the statistics instead of the Existence. This is what the PR is fixing - switching the two.