add custom ip list to cmd

[araskachoi]

Description

testnet command should be able to accept a comma separated string of ip addresses that will be used to generate the proper files for desired number of nodes.

usage:

ethermintd testnet -o ./testnet --keyring-backend=test --ip-addrs=10.10.0.1,192.168.0.1,172.16.0.1 

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #545 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development     #545   +/-   ##
============================================
  Coverage        70.72%   70.72%           
============================================
  Files               42       42           
  Lines             2333     2333           
============================================
  Hits              1650     1650           
  Misses             540      540           
  Partials           143      143           

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[fedekunze]

I think there's a GetStringSlice method

[araskachoi]

fixed