Add "python updates only" cli parameter

[bdragon300]

In this project there are two ways to update documents in database:

• Using mongo pipelines and commands, by_path
• Iterating over collections in python loop and perform update operation for each document. This is python update or by_doc

Selecting one or another way depends on MongoDB current capabilities, but pipelines are always preferred.

Some of converter methods has both implementations, some of them has only by_doc. It depends if a certain converter action can be achieved by executing pipeline or not.

The issue is to add cli parameter for upgrade, downgrade, migrate commands which tells mongoengine-migrate to use by_doc updates only to perform a migration process.