This PR adds a new Storage Backend for Triton's Object Storage - Manta

```
make testacc TEST=./physical/manta
==> Checking that code complies with gofmt requirements...
==> Checking that build is using go version >= 1.9.1...
go generate
VAULT_ACC=1 go test -tags='vault' ./physical/manta -v  -timeout 45m
=== RUN   TestMantaBackend
--- PASS: TestMantaBackend (61.18s)
PASS
ok  	github.com/hashicorp/vault/physical/manta	61.210s
```

Manta behaves differently to how S3 works - it has no such concepts of Buckets - it is merely a filesystem style object store

Therefore, we have chosen the approach of when writing a secret `foo` it will actually map (on disk) as foo/.vault_value

The reason for this is because if we write the secret `foo/bar` and then try and Delete a key using the name `foo` then Manta
will complain that the folder is not empty because `foo/bar` exists. Therefore, `foo/bar` is written as `foo/bar/.vault_value`

The value of the key is *always* written to a directory tree of the name and put in a `.vault_value` file.