[Monitor] Autoscale timezone property should be an enum

https://github.com/Azure/azure-rest-api-specs/blob/master/specification/monitor/resource-manager/microsoft.insights/stable/2015-04-01/autoscale_API.json#L1040

The valid values are C# standard time zone names, of which there are about 136. The problem is that these "Standard Names" are only standard for C#. Python, Java, Ruby, don't have the same names and there is no good way to map to them.

This should be modeled as an enum with all the valid values.

Open specification/monitor/resource-manager/microsoft.insights/stable/2015-04-01/autoscale_API.json at line 1040 and inspect the autoscale timezone property. Confirm the complete set of C# standard time zone names, model those values as an enum, and verify that the generated API definition exposes the enum rather than an unrestricted string.

Written by the indexing model from the issue text.