Documentation update for Service Principals

[jonest]

I was running into some issues running the Execute Sql task in my pipeline and it turns out that this task requires the use of a secret instead of a certificate, as you can see on this line:

gl-vsts-tasks-azure/Common/PowerShell3/VstsAzureHelpers/InitializeFunctions.ps1

Line 110 in 5da8506

(ConvertTo-SecureString $Endpoint.Auth.Parameters.ServicePrincipalKey -AsPlainText -Force))

The error I would see was:

##[debug]Script stack trace:
##[debug]at Initialize-AzureSubscription, D:\a\_tasks\ExecuteSql_a93d571d-f32c-4fe7-a63a-3599dddd5279\1.4.2999\ps_modules\VstsAzureHelpers\InitializeFunctions.ps1: line 108
##[debug]at Initialize-Azure, D:\a\_tasks\ExecuteSql_a93d571d-f32c-4fe7-a63a-3599dddd5279\1.4.2999\ps_modules\VstsAzureHelpers\VstsAzureHelpers.psm1: line 49
##[debug]at <ScriptBlock>, D:\a\_tasks\ExecuteSql_a93d571d-f32c-4fe7-a63a-3599dddd5279\1.4.2999\ExecuteSql.ps1: line 24
##[debug]at <ScriptBlock>, <No file>: line 1
##[debug]at <ScriptBlock>, <No file>: line 22
##[debug]at <ScriptBlock>, <No file>: line 18
##[debug]at <ScriptBlock>, <No file>: line 1
##[debug]Exception:
##[debug]System.Management.Automation.ParameterBindingValidationException: Cannot bind argument to parameter 'String' because it is null.
##[debug]   at System.Management.Automation.ExceptionHandlingOps.CheckActionPreference(FunctionContext funcContext, Exception exception)
##[debug]   at System.Management.Automation.Interpreter.ActionCallInstruction`2.Run(InterpretedFrame frame)
##[debug]   at System.Management.Automation.Interpreter.EnterTryCatchFinallyInstruction.Run(InterpretedFrame frame)
##[debug]   at System.Management.Automation.Interpreter.EnterTryCatchFinallyInstruction.Run(InterpretedFrame frame)
##[debug]   at System.Management.Automation.Interpreter.Interpreter.Run(InterpretedFrame frame)
##[debug]   at System.Management.Automation.Interpreter.LightLambda.RunVoid1[T0](T0 arg0)
##[debug]   at System.Management.Automation.PSScriptCmdlet.RunClause(Action`1 clause, Object dollarUnderbar, Object inputToProcess)
##[debug]   at System.Management.Automation.PSScriptCmdlet.DoEndProcessing()
##[debug]   at System.Management.Automation.CommandProcessorBase.Complete()
##[error]Cannot bind argument to parameter 'String' because it is null.