Update powershell scripts to support fetching publish credentials

[asalvo]

All of our app services have default publishing credentials, so downloading the publishing profile for each is a pain. Our build scripts get the publishing creds and then use them to deploy our code, so I made a added some code to one of the install scripts and got something working.

Between the original check for "ambient" and creating the BASE64 encoded creds, I added the following block which should still support the use of the ambient (env variables).

if($Username -eq "ambient" -or $Username -eq "") {
	Write-Output "No username provided. Fetching publish crendentials from Kudu..."
	$publishProfile = (az webapp deployment list-publishing-credentials --name ${SiteName} --resource-group ${ResourceGroup} --subscription ${SubscriptionId} | ConvertFrom-Json)
	$Username = $publishProfile.publishingUserName
	$Password = $publishProfile.publishingPassword
}