Extract a host_packages method from validate_host

[ekohl]

This separates the logic of getting a list of packages into a separate method. The main motivation for this is that beaker-docker has duplicated the case statement. By providing this method, that reuse it and there's a single place to define this.

[codecov]

Codecov Report

Merging #1729 (ace09dd) into master (cdc17d9) will increase coverage by 0.02%.
The diff coverage is 57.14%.

❗ Current head ace09dd differs from pull request most recent head 252f57e. Consider uploading reports for the commit 252f57e to get more accurate results

@@            Coverage Diff             @@
##           master    #1729      +/-   ##
==========================================
+ Coverage   74.76%   74.78%   +0.02%     
==========================================
  Files          82       82              
  Lines        4846     4851       +5     
==========================================
+ Hits         3623     3628       +5     
  Misses       1223     1223              

Impacted Files	Coverage Δ
lib/beaker/host_prebuilt_steps.rb	73.75% <57.14%> (+0.44%)	⬆️

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