Simplify message queue configuration

[sajith]

Resolves #137: gets rid of some environment variables (SUB_QUEUE, SUB_EXCHANGE, SUB_TOPIC), and use defaults instead.

We do not need a queue name. We do need an exchange name (which would be just "connection") to receive connection/link messages published by SDX-Controller. SUB_TOPIC is not really ever used.

This is stacked on top of #142; please review that PR first.

[coveralls]

coverage: 77.465% (-0.02%) from 77.481%
when pulling 48b6888 on 137.simplify-mq-configuration
into d6a0ac1 on main.

[coveralls]

Pull Request Test Coverage Report for Build 9482732510

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 77.465%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sdx_lc/main.py	0	1	0.0%

Totals
Change from base Build 9482708983:	-0.02%
Covered Lines:	971
Relevant Lines:	1199

---

💛 - Coveralls