If autoscaling is disabled on default_node_pool, those values are set to `0`

…ncipal or MSI blocks

Previously it was possible to use a Service Principal for an AKS Cluster and also give it a
Managed Identity. However due to a breaking change in the AKS API this is no longer possible.

Instead attempting to do this via the API creates an MSI Enabled cluster and ignores the Service
Principal that's specified - as such it's only possible to create a Service Principal or MSI cluster,
not the Service Principal cluster with a Managed Identity attached as before.

…package

…idate since these now exist

…_count` are set to `null` not 0 when undefined

This updates the error message to match the behaviour of the existing resources, where the `_count` fields
can be set to `null` rather than `0` in a coalesce function - for example:

```
locals {
  enable_auto_scale = false
  fixed_count = 2
  min_count   = 1
  max_count   = 4
}

resource "azurerm_kubernetes_cluster" "test" {
  # ...

  default_node_pool {
    enable_auto_scaling = local.enable_auto_scale
    node_count          = local.enable_auto_scale ? null : local.fixed_count
    min_count           = local.enable_auto_scale ? local.min_count : null
    max_count           = local.enable_auto_scale ? local.max_count : null
  }
}
```

…S API

Due to a breaking change in the AKS API - it's no longer possible to create
mixed-mode clusters (e.g. using both a Service Principal and a Managed Identity).

This commit attempts to correctly message this behaviour, whilst trying to ensure
we can adequately capture this behaviour within Terraform

…pool tags

…efault node pool

Also fixing a bug where min/max count would be unmodified during updates

…SI are used

…an `0`

When coalescing between these values - this should be set to `null` rather
than `0` - and fixes the error message which caused the initial confusion.
Whilst setting this field to `0` is convenient - it breaks validation for
when auto-scaling is enabled - as such `0` shouldn't be an allowed value
for `min_count` or `max_count` (since node pools can't be scaled to `0` in
the API)

Instead users can coaslesce this via:

```
locals {
  enable_auto_scale = false
  fixed_count = 2
  min_count   = 1
  max_count   = 4
}

resource "azurerm_kubernetes_cluster_node_pool" "test" {
  # ...
  enable_auto_scaling = local.enable_auto_scale
  node_count          = local.enable_auto_scale ? null : local.fixed_count
  min_count           = local.enable_auto_scale ? local.min_count : null
  max_count           = local.enable_auto_scale ? local.max_count : null
}
```

Originally made by @adback03 in hashicorp#6357

…unt`/`max_count` being null

…ther optional props