Update managed disk docs with correct sizes & add Ultra SSD type

[katbyte]

also removed import tests & tidied tests

Fixes #2115

[katbyte]

tests pass:

gofmt -w $(find . -name '*.go' |grep -v vendor)
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./azurerm -v -test.run=TestAccAzureRMManagedDisk -timeout 180m -ldflags="-X=github.com/terraform-providers/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccAzureRMManagedDisk_empty
--- PASS: TestAccAzureRMManagedDisk_empty (72.24s)
=== RUN   TestAccAzureRMManagedDisk_zeroGbFromPlatformImage
--- PASS: TestAccAzureRMManagedDisk_zeroGbFromPlatformImage (77.49s)
=== RUN   TestAccAzureRMManagedDisk_import
--- PASS: TestAccAzureRMManagedDisk_import (401.24s)
=== RUN   TestAccAzureRMManagedDisk_copy
--- PASS: TestAccAzureRMManagedDisk_copy (131.90s)
=== RUN   TestAccAzureRMManagedDisk_fromPlatformImage


--- PASS: TestAccAzureRMManagedDisk_fromPlatformImage (79.70s)
=== RUN   TestAccAzureRMManagedDisk_update
--- PASS: TestAccAzureRMManagedDisk_update (80.76s)
=== RUN   TestAccAzureRMManagedDisk_encryption
--- PASS: TestAccAzureRMManagedDisk_encryption (218.39s)
=== RUN   TestAccAzureRMManagedDisk_NonStandardCasing
--- PASS: TestAccAzureRMManagedDisk_NonStandardCasing (79.30s)
=== RUN   TestAccAzureRMManagedDisk_importEmpty_withZone
--- PASS: TestAccAzureRMManagedDisk_importEmpty_withZone (69.74s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	1210.794s

[WodansSon]

@katbyte LGTM

[tombuildsstuff]

LGTM 👍

[ghost]

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