Vagrant private_key_path won't show in ssh-config

Question

I have the following vagrantfile

 Vagrant.configure("2") do |config|
     config.vm.box = "ubuntu/xenial64"
     config.ssh.username = "ndp"
     config.ssh.keys_only = true
     config.ssh.insert_key = true
     config.ssh.paranoid = true
     config.ssh.private_key_path = File.expand_path("./keys/id_rsa",File.dirname(__FILE__))
     config.vm.network "public_network", bridge: "en0: Wi-Fi (AirPort)", ip: "192.168.1.200"
     config.vm.synced_folder "./", "/vagrant_data/"

     config.vm.provider "virtualbox" do |vb|
        # vb.gui = true
        vb.memory = "2048"
    end
end

However the ssh-config shows:

vagrant ssh-config
Host default
  HostName 127.0.0.1
  User ndp
  Port 2222
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  PasswordAuthentication no
  IdentitiesOnly yes
  LogLevel FATAL

According to other stack overflow answers I should be seeing a line including the path to my IdentityFile. When the path in private_key_path is wrong Vagrant throws the correct error about the key file not existing.

Moreover, trying to vagrant ssh to my box I get prompted for a password. However ssh-ing without vagrant works

ssh [email protected] -i keys/
Welcome to Ubuntu 16.04.1 LTS (GNU/Linux 4.4.0-45-generic x86_64) ....

Finally, trying to ssh with VAGRANT_LOG set to info shows that it's trying to execute the following:

INFO ssh: Invoking SSH: ssh ["[email protected]", "-p", "2222", "-o", "Compression=yes", "-o", "DSAAuthentication=yes", "-o", "LogLevel=FATAL", "-o", "StrictHostKeyChecking=no", "-o", "UserKnownHostsFile=/dev/null", "-o", "IdentitiesOnly=yes"]

Which makes zero sense as the following connects perfectly

ssh [email protected] -p 2222 -o DSAAuthentication=yes -o IdentitiesOnly=yes -i keys/id_rsa

I've tried removing the keys from .vagrant/ but it didn't work. This behaviour breaks even basic operations such as halt and reload since they are depended on ssh I'm running Vagrant 1.8.4 on El Capitan 10.11

Since the purpose of the VM is to host a portable dev environment it doesn't make sense to give it a static ip. Ideally I'd like to have the provisioning script get the *.pub key from the keys dir and

cat $key >>~/.ssh/authorized_keys

So devs can easily submit their own keys to the machines, however this is not possible since vagrant won't load the keys

Any ideas?