Change function names to suit POSH conventions

SetSubscription -> Set-Subscription
CheckVariablesAreDefined -> Confirm-VariablesAreDefined
New-AzureStorageAccountIfNotExists -> CreateAzureStorageAccountIfNotExists
CreateAzureStorageContainerIfNotExists -> New-AzureStorageContainerIfNotExists
UploadBlobs -> Set-AzureStorageBlobs
CreateHDInsightClusterIfNotExists -> New-HDInsightClusterIfNotExists