Linux profile should be optional for AKS creation

[metacpp]

Bug Report

• Import package: .../services/containerservice/mgmt/2018-03-31/containerservice
• SDK version: 18.0.0
• output of go version: go1.9 darwin/amd64

• What happened?
  Try to create AKS without linuxProfile with below validation error message:

Error: Error applying plan:

1 error(s) occurred:

* azurerm_kubernetes_cluster.test: 1 error(s) occurred:

* azurerm_kubernetes_cluster.test: containerservice.ManagedClustersClient#CreateOrUpdate: Invalid input: autorest/validation: validation failed: parameter=parameters.ManagedClusterProperties.LinuxProfile.AdminUsername constraint=Null value=(*string)(nil) details: value can not be null; required parameter

Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.

• What did you expect or want to happen?
  From Azure portal, there's no way for end user to set the linuxProfile. The API should be consistent with Azure portal.
  This caused below issue escalated by customer:
  Can't import portal-created AKS clusters hashicorp/terraform-provider-azurerm#1806

[vladbarosan]

@metacpp please track any updates on the linked specs issue to avoid duplicates. Also note that if a change is required that is going to be a breaking change it should be in this week or early next week as we do breaking change releases at the end of the month.