keyring_vault's timeout should be configurable

Currently there is hardcoded 5m timeout when keyring_vault communicates with Vault server. This can be problematic when Vault server is not reachable - it will cause server to block. This is not sever issue for now - as the connection to Vault server is established on the server start-up and only one key (Innodb Master Key) is retrieved from the server. However, as more encryption features will be added to the server the more keys will get fetched during server's runtime and 5m pauses are unacceptable.