How to set pm_password for pxe_ssh

Issue

• How to specify RSA Private key in instackenv.json file for pxe ssh .

  "nodes": [
    {
      "arch": "x86_64",
      "pm_user": "root",
      "pm_addr": "XX.XX.XX",
      "pm_password": "xxxx",
      "pm_type": "pxe_ssh",
      "mac": [
        "xx:xx:xx:xx:xx:xx"
      ],
      "cpu": "x",
      "memory": "16384",
      "disk": "500"
    }
  ]

• pm_password does not accept the private key in proper format .