WIP: Support for @SUMMONVARNAMES

[doodlesbykumbi]

What does this PR do?

@SUMMONVARNAMES makes this possible, which avoids the multiline issues of @SUMMONENVFILE.

$ summon -p keyring.py -D env=dev env SUMMONVARNAMES=@SUMMONVARNAMES sh << EOL
docker run \$(printenv SUMMONVARNAMES | xargs printf -- '--env %s ' | xargs) deployer
EOL

• What's changed? Why were these changes made?
• How should the reviewer approach this PR, especially if manual tests are required?
• Are there relevant screenshots you can add to the PR description?

What ticket does this PR close?

Resolves #199

Checklists

Change log

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a CHANGELOG update

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• This PR includes new unit and integration tests to go with the code changes, or
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[ryanneufeld]

This would be fantastic to have

[ryanneufeld]

@jtuttle Any way this could be added?

[jtuttle]

@doodlesbykumbi Is this ready for review? I see it still says WIP.

[doodlesbykumbi]

@jtuttle This PR was and to some extent still is really just experimental. We never got to a point where we found an approach that we were entirely happy with.