Use request connection id when saving to db

[congwang09]

Resolves: https://github.com/atlanticwave-sdx/sdx-controller/issues/256

Use connection ID instead of hard coded db key name.

Also if we cannot generate a graph, it should not be 400, because it's not due to bad request. Would 500 make sense?

[coveralls]

Pull Request Test Coverage Report for Build 8695242743

Details

• 7 of 11 (63.64%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on use-connection-id-for-db at 50.919%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sdx_controller/handlers/connection_handler.py	0	1	0.0%
sdx_controller/controllers/connection_controller.py	7	10	70.0%
<!--	Total:	7	11	63.64%	-->

Totals
Change from base Build 8664224346:	50.9%
Covered Lines:	708
Relevant Lines:	1421

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💛 - Coveralls

[congwang09]

When no 'id' in body, should the newly generated 'id' be also added to the body?

Sure. Added that. Actually id is a required field enforced by swagger, so this probably is not very useful anyway.

[YufengXin]

it's good that you catch it, maybe also adding a log somewhere earlier.

Looks like I can't review it again? @sajith pls review/approve.