[admin] make SolidusAdmin opt-in while installing solidus

[elia]

Summary

Ensure SolidusAdmin is really opt-in during the installation.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

Merging #5400 (a5372d3) into nebulab/admin (1288f9c) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           nebulab/admin    #5400   +/-   ##
==============================================
  Coverage          88.77%   88.77%           
==============================================
  Files                609      609           
  Lines              14762    14762           
==============================================
  Hits               13105    13105           
  Misses              1657     1657           

Files	Coverage Δ
...ib/generators/solidus/install/install_generator.rb	49.63% <100.00%> (ø)

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