How do I configure SSH keys in a Vagrant multi-machine setup?

I have 4 VMs in my Vagrantfile - 3 application servers and an Ansible control host.

I only use Vagrant to create the VMs as I provision them manually from the ansible control host because I am still creating/editing the ansible scripts.

I can do vagrant ssh ansible and vagrant ssh app1/2/3 etc. but when I try to do ansible-playbook oracle.yml from the Ansible control host, SSH fails with

fatal: [192.168.60.10]: UNREACHABLE! => {"changed": false, "msg": "SSH encountered an unknown error during the connection. We recommend you re-run the command using -vvvv, which will enable SSH debugging output to help diagnose the issue", "unreachable": true}

I can successfully ssh from the Ansible VM to the Oracle VM using user vagrant and password vagrant.

The key parts of my Vagrantfile are:

config.ssh.insert_key = falseconfig.vm.define "db" do |db|    db.vm.box = "boxcutter/ol67"    db.vm.hostname = "oracle-vm"    db.vm.network "forwarded_port", guest: 22, host: 2201, id: "ssh", auto_correct: false    db.vm.network "forwarded_port", guest: 1521, host: 1521    db.vm.network "private_network", ip: "192.168.60.10"    db.vm.provider "virtualbox" do |v|        v.name = "oracle-vm"        v.linked_clone = true        v.memory = 2048        v.cpus = 2    endend#Optional ansible control machine for Windows usersconfig.vm.define "ansible", autostart: false do |ansible|    ansible.vm.box = "williamyeh/ansible"    ansible.vm.hostname = "ansible-vm"    ansible.vm.network "forwarded_port", guest: 22, host: 2204, id: "ssh", auto_correct: false    ansible.vm.network "private_network", ip: "192.168.60.50"    ansible.vm.provider "virtualbox" do |v|        v.linked_clone = true    end    #Mount the project directory on the guest so we can run the playbooks from there    ansible.vm.synced_folder ".", "/data/ansible", create: trueend

What do I need to put in the Vagrantfile to allow the Ansible VM to connect to the other VMs without requiring a password or extra manual steps after vagrant up?

This is just for development testing on a private network on developers PCs so security is not really an issue and comes second to ease of implementation and smooth user experience.