Allow specifying webhooks to get installer event notifications

[bk201]

Relate issue
harvester/harvester#469

With this change, the user can add webhooks in the config and the installer will send HTTP
requests to custom URLs on some predefined events.

The user can add webhooks under the install key of the harvester configuration file:

The following is a configuration example:

install:
  webhooks:
    - event: SUCCEEDED
      method: GET
      url: http://10.100.0.100/cblr/svc/op/nopxe/system/{{.Hostname}}
    - event: STARTED
      method: GET
      url: https://10.100.0.100/started/{{.Hostname}}
      insecure: true
      basicAuth:
        user: admin
        password: p@assword
    - event: FAILED
      method: POST
      url: http://10.100.0.100/record
      headers:
        Content-Type:
           - 'application/json; charset=utf-8'
      payload: |
        {
          "host": "{{.Hostname}}",
          "device": "hd"
        }

Explanation of the fields:

• event: Event type to trigger HTTP action on the webhook

  • STARTED: The installer just begins to perform installation.
  • SUCCEEDED: The installation completes without errors.
  • FAILED: The installation is failed.

  Example:

  event: SUCCEEDED

• method: HTTP method

  Example:

  method: GET

• url*: The URL to send the HTTP request.

  url: http://mycompany.com

• insecure: Do not verify server's certificate if true. Default is false.

  Example:

  insecure: true

• basicAuth: Use 'Basic' HTTP Authentication.

  Example:

  basicAuth:
    user: admin
    password: p@ssword

• headers: Include custom headers in the HTTP requests.

  Example:

  headers:
    Content-Type:
      - 'application/json; charset=utf-8'

  Above configuration will includes Content-Type: application/json; charset=utf-8 in the HTTP request.

  NOTE: Headers like Content-Length are automatically included.

• payload*: The payload data to be sent with the request.

  Example:

  payload: |
    {
      "host": "{{.Hostname}}",
      "device": "hd"
    }

  NOTE: The user might need to set the correct Content-Type header with the headers configuration to make the server accept the request.

* The values for url and payload can be written in Go template, the following variables will be substituted with appropriate values of the node that is being installed:

• Hostname: the hostname of the node.
• MACAddr: the MAC address of the management interface
• IPAddrV4: the IPv4 address of the management interface
• IPAddrV6: the IPv6 address of the management interface

Additional information

• The installer can send multiple requests for a single event. But if one of them fails, the remaining requests will not be sent.
• The installer will retry 3 times with a 5 seconds-sleep if the request is timed out.

[guangbochen]

@bk201 can u please rebase the PR and take a look at the above comments, thanks.

[bk201]

@bk201 can u please rebase the PR and take a look at the above comments, thanks.

Rebased!

[gitlawr]

LGTM