[parser] nested json records will now have optional keys applied

[ryandeivert]

to @jacknagz , @mime-frame
cc @airbnb/streamalert-maintainers
resolves: #166
size: small

changes

• Nested json records that get extracted via json_path will now have optional_top_level_keys applied.
• Removing the need to call update on a schema to add the default value, and instead logging an error if the key does not exist in the schema. This informs the user that the optional_top_level_keys should also be declared as part of the schema in logs.json.

todo

• A future PR is to come with a note added to the docs about the need for optional_top_level_keys to be declared in the schema.

[jacknagz]

Change looks good - please add a unit test case which uses optional_top_level_keys as well as json_path to verify

[ryandeivert]

@jacknagz amended with new unit test. PTAL

[jacknagz]

💯