Validate that helm chart values exist

[patrykkulik-microsoft]

This PR is for https://dev.azure.com/msazuredev/AzureForOperators/_sprints/taskboard/Turtle/AzureForOperators/Germanium/CY24Q1/2Wk/2Wk09%20(Jan%2014%20-%20Jan%2027)?workitem=1085390

This story is to make sure that :

1. whenever the user provided default_values parameter for the helm chart, we use it preferentially over the values.yaml in the chart itself. - as far as I can tell we have already been doing this everywhere other than creating a schema. I fixed this.
2. we mandate that either a values.yaml exists or a default_values parameter is provided. This was mostly done in our repo in the get_defaults() method. Problem was that this method was called after validation of the helm template. Unfortunately, in the case where the values.yaml file is missing, the helm template command does not provide a particularly good error. I moved some logic around to add a new function to make sure that a value.yaml or default_values exist. Otherwise, it will throw a useful error.
3. Added some unit tests to test this functionality.