Keep a more flat property list

[j-windsor]

The mig modules (and likely other modules in this repo) has several groups of properties in objects that could easily be flattened to root level properties. For example:

• health_check: In my configuration, because I wanted to set 3 properties to custom values, I had to set 9 additional properties to default values that I could have just left blank if they were root level properties with defaults.
• update_policy: This property has no default values, but there are many properties here that could use default values matching the default values in the Cloud Console.

Keeping a more flat configuration with most properties right on the root of the module make it so you can effectively set defaults, makes the module more compatible with IDE autocomplete tools, and more easily documentable using TF descriptions.