Fix schema utils handling of nested arrays

[cognifloyd]

Fixed schema utils to more reliably handle schemas that define nested arrays (object-array-object-array-string) as discovered in some of the ansible installer RBAC tests. This includes a test that reproduced the error so we don't hit this again.

Closes #5684

[cognifloyd]

I added a reproducer test for #5684 in 3413a82 which you can see failing in CI here:
https://github.com/StackStorm/st2/runs/7485827132?check_suite_focus=true#step:15:1223

======================================================================
2) ERROR: test_nested_schemas (tests.unit.test_json_schema.JSONSchemaTestCase)
----------------------------------------------------------------------
   Traceback (most recent call last):
st2common/tests/unit/test_json_schema.py line 450 in test_nested_schemas
      instance=instance, schema=TEST_SCHEMA_6, cls=validator, use_default=True
st2common/st2common/util/schema/__init__.py line 370 in validate
      instance = assign_default_values(instance=instance, schema=schema)
st2common/st2common/util/schema/__init__.py line 256 in assign_default_values
      instance=array_instance, schema=schema_items
st2common/st2common/util/schema/__init__.py line 249 in assign_default_values
      array_instance = instance.get(property_name, None)
AttributeError: 'list' object has no attribute 'get'

Then I refactored assign_default_values to separate object vs array handling in 9000680 which passes all the tests including the reproducer. With the array and object stuff smashed together, I was having a hard time understanding which pieces were supposed to handle which types. So, splitting that into separate type-specific functions clarified that. It should be much easier to follow the assign_default_values logic now.

I also left a few TODOs where I noticed some schema features like additionalProperties that this code does not handle.

edit: well, that makes this bug fix sound more straightforward than it was. See #5684 (comment) for more on the other things I tried before coming up with this.

[amanda11]

LGTM, and great to have the test case so we don't suffer problem again.

[nzlosh]

LGTM