Remove MongoDB internal _id from the user response and change status code for graph validation

[italovalcy]

Fixes #260 Fixes #34

Description of the change

• The error presented on Issue #260 was related to the serialization of the internal MongoDB attribute _id. That issue could be fixed by changing the Encoder to handle ObjectID as a string (as suggested here). However, after analyzing the full picture I understood it was better to remove that field from the response to the user and return only the actual connection data
• I've also augmented the getconnection_by_id with validation for connections not found, to make it comply with the swagger spec (https://github.com/atlanticwave-sdx/sdx-controller/blob/main/sdx_controller/swagger/swagger.yaml#L189-L190)
• I also took this opportunity to enhance the HTTP status code returned by graph PCE return to return 424 (dependency failed) instead of 500 (Internal server error)
• Finally, adjusted the tests to reflect the new get connection status code and added a success test in the end

[congwang09]

Do we need to update the part where the encoder.py handles the JSON request?

[italovalcy]

Do we need to update the part where the encoder.py handles the JSON request?

I realized that internal _id attribute was actually useful for other parts of the code. I'm reviewing the approach now to make sure it will not break compatibility.

[coveralls]

Pull Request Test Coverage Report for Build 8855512540

Details

• 3 of 4 (75.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 51.046%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sdx_controller/handlers/connection_handler.py	0	1	0.0%
<!--	Total:	3	4	75.0%	-->

Totals
Change from base Build 8756604524:	0.1%
Covered Lines:	710
Relevant Lines:	1422

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💛 - Coveralls