[Python schema] Support python avro schema set default value.

[congbobo184]

Motivation

now python avro schema don't support set default value, it led to the python schema can't update.

implement

1. add required field to control the type of schema can set null.
2. add required_default filed to control to control the schema has default attribute wether or not.
3. add default field to control the default value of schema.

Verifying this change

Add the tests for it

Does this pull request potentially affect one of the following parts:
If yes was chosen, please highlight the changes

Dependencies (does it add or upgrade a dependency): (no)
The public API: (no)
The schema: (no)
The default values of configurations: (no)
The wire protocol: (no)
The rest endpoints: (no)
The admin cli options: (no)
Anything that affects deployment: (no)

[congbobo184]

@BewareMyPower could you please review this pr? thanks :)

[BewareMyPower]

It looks like there're still some problems with the implementation.

File "/tmp/schema_test.py", line 810, in test_avro_required_default
    "default": None
AssertionError: {'fields': [{'default': None, 'type': ['null', 'int'], 'name': 'a'}, {'default': [truncated]... != {'fields': [{'default': None, 'type': ['null', 'int'], 'name': 'a'}, {'default': [truncated]...

[codelipenghui]

mark as release/2.7.2 since this is a bug when the users using the schema in the Python client.