The repository 'http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release' is not signed

Description

1. apt-get update
   Ign:1 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 InRelease
   Ign:2 http://repo.mongodb.org/apt/debian wheezy/mongodb-org/3.2 InRelease
   Hit:3 http://mirrors.digitalocean.com/ubuntu bionic InRelease
   Get:4 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
   Hit:5 http://ppa.launchpad.net/certbot/certbot/ubuntu bionic InRelease
   Ign:6 https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.0 InRelease
   Hit:7 https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.0 Release
   Hit:8 http://mirrors.digitalocean.com/ubuntu bionic-updates InRelease
   Hit:9 http://ppa.launchpad.net/ondrej/php/ubuntu bionic InRelease
   Hit:10 http://archive.ubuntu.com/ubuntu bionic InRelease
   Hit:11 http://mirrors.digitalocean.com/ubuntu bionic-backports InRelease
   Get:12 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release [3457 B]
   Hit:13 http://repo.mongodb.org/apt/debian wheezy/mongodb-org/3.2 Release
   Get:14 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release.gpg [801 B]
   Ign:14 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release.gpg
   Reading package lists... Done
   W: GPG error: http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY BC711F9BA15703C6
   E: The repository 'http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release' is not signed.
   N: Updating from such a repository can't be done securely, and is therefore disabled by default.
   N: See apt-secure(8) manpage for repository creation and user configuration details

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)