azure - fix app service plan scale out

[aluong]

fix #4734
fix #4606

• Updated examples to show how to increase number of workers (scale out)
• Make size a required parameter
• count is still optional and if you attempt to resize from a tier with more workers than the tier you are scaling down to, then an exception will be thrown.

Example: Start from B1:2 -> F1

2019-09-04 22:53:00,365: custodian.azure.AzureBaseAction:ERROR Failed to resize cctest-appserviceplan-linux. Inner exception: {'additional_properties': {'Message': 'Number of Web workers exceeds the maximum allowed for the hosting plan.', 'Details': [{'Message': 'Number of Web workers exceeds the maximum allowed for the hosting plan.'}, {'Code': 'Conflict'}, {'ErrorEntity': {'ExtendedCode': '03002', 'MessageTemplate': 'Number of Web workers exceeds the maximum allowed for the hosting plan.', 'Parameters': [], 'Code': 'Conflict', 'Message': 'Number of Web workers exceeds the maximum allowed for the hosting plan.'}}], 'Code': 'Conflict', 'Target': None, 'Innererror': None}, 'error': None}