Add archived connection api

[congwang09]

Resolves: #238

This PR adds an API to query archived connections for a specific service ID. It may not make sense to be able to query all archived connections at this point, because there can be plenty of connections, resulting a huge response.

[coveralls]

Pull Request Test Coverage Report for Build 11241646530

Details

• 4 of 12 (33.33%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 58.166%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sdx_controller/controllers/l2vpn_controller.py	3	7	42.86%
sdx_controller/handlers/connection_handler.py	1	5	20.0%

Totals
Change from base Build 11184666373:	-0.2%
Covered Lines:	1088
Relevant Lines:	1927

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💛 - Coveralls

[YufengXin]

in swagger.yaml, is this necessary for xml?

         application/xml:
          schema:
            $ref: '#/components/schemas/connection'

---

[congwang09]

in swagger.yaml, is this necessary for xml?

         application/xml:
          schema:
            $ref: '#/components/schemas/connection'

Yeah I think this is not needed, it exists for other APIs, for example: /l2vpn/1.0/{service_id} here. I'll create a separate PR to test and clean this up.