Fix migrating away from raft storage

[briankassouf]

% cat migrate.hcl
storage_source "raft" {
  path = "/Users/brian/vault-raft/"
  node_id = "node2"
}

storage_destination "consul" {
  address = "127.0.0.1:8500"
  path    = "vault"
}

% vault operator migrate -config migrate.hcl
2020-03-05T18:16:48.881-0800 [WARN]  appending trailing forward slash to path
2020-03-05T18:16:48.883-0800 [INFO]  copied key: path=core/audit
2020-03-05T18:16:48.884-0800 [INFO]  copied key: path=core/auth
2020-03-05T18:16:48.884-0800 [INFO]  copied key: path=core/cluster/feature-flags
2020-03-05T18:16:48.886-0800 [INFO]  copied key: path=core/cluster/local/info
2020-03-05T18:16:48.886-0800 [INFO]  copied key: path=core/hsm/barrier-unseal-keys
2020-03-05T18:16:48.886-0800 [INFO]  copied key: path=core/keyring
2020-03-05T18:16:48.887-0800 [INFO]  copied key: path=core/leader/ebf0c339-1004-2eef-3818-9fc2c978dce3
2020-03-05T18:16:48.887-0800 [INFO]  copied key: path=core/local-audit
....