add command to list deployments for a given slot for cli 2.0
The older version of the azure cli had "azure site deployments list" which provided a list of the commits for the repo linked to source control. It did it (and this is important) by slot. So you could see which deployment was currently active.
I've confirmed through a support request that this command is only available through Get-AzureWebsiteDeployment now. I don't use PowerShell and I don't believe it runs on the mac.
This command is very handy for seeing which deployment is live, for production AND staging slots. After a swap it is especially useful.
