Databricks API allows provisioning using any case, but is case-sensitive for cluster login

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We've recently added support for Databricks Clusters to Terraform, which makes use of version `2018-04-01` of the Azure API.

During the initial investigation into this resource we noted that Databricks Clusters can be provisioned either using a `Standard` or `Premium` SKU's - as such we've added support for these. After shipping this functionality to customers [we've identified an issue](https://github.com/terraform-providers/terraform-provider-azurerm/issues/2214) with the casing of the SKU - where whilst it's possible to provision a Databricks Cluster using a `Standard` or `Premium` SKU using the Resource Manager API, it's not possible to log into the cluster if it's provisioned using either of these casings.

By contrast - if a Databricks Cluster is provisioned using a SKU of `standard` or `premium` (note the casing) - it's possible to log in to the Databricks Cluster using Azure AD. Whilst I can understand being case-sensitive for the Sku/Enum - in this case it seems odd to allow this to be provisioned using any case but require that it's lower-cased to be able to log into the cluster.

Given it's possible to provision this Cluster using either casing for the SKU - would it be possible to update this so that it's possible to login to the cluster using either casing for the SKU too? Other API's in Azure require that the casing is Title Case rather than lower cased - so it seems odd that the lower-cased version is required.

Thanks!