provider/softlayer: Implement softlayer_provisioning_hook Resource

[danielcbright]

Implement Resource: softlayer_provisioning_hook

This issue will track the completion of the _provisioninghook resource type requirement for the SoftLayer Provisioner.

Tracking Checklist

• [x] Gather all resource requirements for this resource from SUN, compare with existing AWS TF configurations and create an example configuration in the description of this issue.
• [ ] Implement Resource Arguments defined in the example configuration provided in this issue.
• [x] Implement any required changes to the softlayer-go API, including enhancements and tests. Please follow all contribution guidelines listed in the main README, all updates should be made via PR to https://github.com/TheWeatherCompany/softlayer-go which will be code-reviewed and then merged, changes will eventually be merged upstream.
• [ ] Create acceptance tests following the TF Acceptance Test Guidelines. The AWS provider has good examples of acceptance tests.
• [ ] Open PR and code-review for the Terraform coding.
• [ ] Perform QA testing of the code, basically make sure it does what it's meant to do.
• [ ] Update the website documentation for the SoftLayer Provider following the instructions in the Terraform Website tree.
• [ ] When all other required resources are completed, perform UAT testing of the complete suite of resources.
• [ ] Fix any "show-stopper" bugs that come up as a result of UAT testing, update documentation as needed.
• [ ] Once UAT testing is complete and it is accepted, create a PR to Hashicorp and close this issue.

Example configuration:

resource softlayer_provisioning_hook "test_provisioning_hook" {
    name = "srinivas-hook-july21"
    uri = "https://raw.githubusercontent.com/srinivas-dandibhotla/slvm/master/apt-vm-init-vim.sh"
}

[renier]

We are also going to need to modify virtual_guest to accept a provisioning hook label as a parameter.

[riethm]

After some discussion, we no longer feel that adding a 'provisioning_hook_label' field to the virtual_guest resource adds sufficient value to offset the complexity in doing so.

Some factors that weigh into this thinking:

1. SoftLayer does not maintain the relationship between provisioning hook 'object' and virtual guest, so there is no way to read the information back during a refresh, or an update to an existing state.
2. Updating the provisioning hook directly in SoftLayer could result in ambiguous behavior relative to the user's expectation. E.g., Upon modifying the URI of a provisioning hook, or deleting it entirely from SoftLayer, what action(s) should be taken on virtual_guest resources that previously referenced that provisioning hook?
3. There are existing trivial alternatives for passing a common URI to a virtual_guest resource
   1. Terraform variables
   2. Interpolation of the URI variable from a provisioning_hook resource defined within the same configuration, e.g.,:

resource "softlayer_provisioning_hook" "my_hook" {
    name = "mike_custom_script"
    uri = "https://something"
}

resource "softlayer_virtual_guest" "guest1" {
    …
    post_install_script_uri = "${softlayer_provisioning_hook.my_hook.uri}"
    …
}