Replace user with openssh_keypair

On Fedora 33 user: doesn't create id_rsa.pub, so next task fails as

TASK [Slurp pub key]
fatal: [test]: FAILED! => {
    "changed": false
}

MSG:

file not found: /home/sgolovat/.ssh/id_rsa.pub

This patch replaces user: with openssh_keypair: to have more control
over ssh key pair creation

Change-Id: I6b303c13957f905aae0a04ea99cf53ceb9684de2

tests/prepare-test-host.yml

@@ -44,11 +44,9 @@
         mode: "0700"
 
     - name: Create ssh key pair
-      user:
-        name: "{{ ansible_user }}"
-        generate_ssh_key: true
-        ssh_key_bits: 2048
-        ssh_key_file: "{{ ansible_user_dir }}/.ssh/id_rsa"
+      openssh_keypair:
+        path: "{{ ansible_user_dir }}/.ssh/id_rsa"
+        size: 2048
 
     - name: Slurp pub key
       slurp: