AzureStackHCI Image Create Should Validate --os-type Casing

**Related command**

`az azurestackhci image create`

**Describe the bug**

The resource provider uses an enum for OS type which expect specific casing ('Windows' or 'Linux'). If you attempt to create an image with --os-type 'windows' or 'linux', the RP returns an error message that is difficult for customers to interpret.

**To Reproduce**

If you attempt to create an image with --os-type 'windows' or 'linux', the RP returns an error message that is difficult for customers to interpret.

**Expected behavior**

The Azure CLI should validate the casing of the --os-type parameter or correct it on submission

**Environment summary**

{
"experimental": true,
"extensionType": "whl",
"name": "azurestackhci",
"path": "C:\\Users\\x\\.azure\\cliextensions\\azurestackhci",
"preview": false,
"version": "0.2.6"
},

**Additional context**