provider/softlayer: Implement softlayer_network* Resource

[danielcbright]

Implement Resource: softlayer_network*

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

• Requirements Gathering Doc

Tracking Checklist

• 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.
• 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 Terraform Configuration [WIP]:

WIP