Tidy up the data model

## Background
Our current data model has issues which when fixed could made maintaining the project much easier in the future.

## Feature
Tidy up the model as soon as possible so we won't need to do many complicated migrations between versions.

The list below is open for extension. Please, add things you would like to improve in out data model there.

## Proposed Solution
1. `OozieSchedule` object has fields that might be better placed elsewhere:
a. `mappingTablePattern` is better to be placed in `MappingTable`.
b. `rawFormat` is better to be placed in `Dataset`
2. If we put `rawFormat` into `Dataset` we might consider putting format-specific options there as well (rowtag, delimiter, etc.). Possibly as a String to String map so we would be able to support arbitrary format-specific options. For COBOL the number of options is so big that adding an attribute for each of them might be too much.
3. `parent` fields in entities has type of `MenasReference`. `MenasReference` has an optional `collection` parameter which should be None in the most imaginable cases. The exception is `attachment` collection. Possible solutions:
a. Remove `collection` from `MenasReference`.
b. Or (maybe better) reuse `MenasReference` in `Attachment`.