R4R: Add --chain-id to testnet command

[alessio]

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[cwgoes]

utACK

[greg-szabo]

Awesome, this is exactly what I need.

[codecov]

Codecov Report

Merging #2749 into develop will decrease coverage by 0.04%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #2749      +/-   ##
===========================================
- Coverage    56.89%   56.85%   -0.05%     
===========================================
  Files          155      155              
  Lines         9751     9755       +4     
===========================================
- Hits          5548     5546       -2     
- Misses        3825     3831       +6     
  Partials       378      378