Specifying ssh key for delegate_to target

[Gin]

Hi,

I am trying to provision a couple of hosts:

[masters]
master-1

[nodes]
node-1

Each host has an individual private key used to ssh to that particular server. I define ansible_ssh_private_key_file and store private keys on localhost: 

"./deployment/{{inventory_hostname}}/id_rsa"

In this example, there are two keys at the following locations:

./deployment/master-1/id_rsa
./deployment/node-1/id_rsa

I would like to execute a slurp task while on node-1. The task reads the contents of a file that resides on master-1:

- name: Read remote
  slurp:
    src: "/remote/file"
  register: remote_value
  run_once: true
  delegate_to: "{{ groups['masters'][0] }}"

At the moment the task fails saying it is unable to ssh to master-1. And that is correct, considering that ansible_ssh_private_key_file is resolved to ".../node-1/id_rsa".
How can I specify private key location for delegate_to: master-1 (.../master-1/id_rsa)?

Thanks,
Gintas