AZ CLI: Add name availability check to az bot create

## Tool
Azure CLI

## Describe the bug
az bot create should use the [`get_check_name_availability`](https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/botservice/azure-mgmt-botservice/azure/mgmt/botservice/operations/bots_operations.py#L440) API available for validating the az bot create name. 


[bug]
