[AKS] Change Cmdlet names of Get/New/Remove/Set-AzAks

[erich-wang]

Description of the new feature

Change of names for Get-AzAks/New-AzAks/Remove-AzAks/Set-AzAks to
Get-AzAksCluster/New-AzAksCluster/Remove-AzAksCluster/Set-AzAksCluster, and add original names as alias to avoid breaking change.