constructor ({ vaultFactory, tokenRenewThreshold, logger, defaultVaultMountPoint, defaultVaultRole }) {
    super({ logger })
    this._vaultFactory = vaultFactory
    this._clients = new Map()
    this._tokenRenewThreshold = tokenRenewThreshold
    this._defaultVaultMountPoint = defaultVaultMountPoint
    this._defaultVaultRole = defaultVaultRole
  }
Create Vault backend.
@param {Object} vaultFactory - arrow function to create a vault client.
@param {Number} tokenRenewThreshold - tokens are renewed when ttl reaches this threshold
@param {Object} logger - Logger for logging stuff.