Update to allow a user to not specify a secret name

[begoldsm]

This was behavior we had in hyak, but lost due to limitations in
AutoRest. Adding it back.

[azurecla]

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[markcowl]

on demand run here: http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-demand/727/

signing run here: http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-sign/397/

[markcowl]

@begoldsm merge conflicts, please update

[markcowl]

@azuresdkci retest this please

[begoldsm]

on demand run: http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-demand/735/

signing run here: http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-sign/402/