Refactor built-in Dockerfile and fix_ssh()

[jay7x]

Changes:

• Reduce layers amount in the image generated from the built-in Dockerilfe (every RUN produces another layer)
• Squash multiple sed invocations into one
• Sync sshd_config changes in the built-in Dockerfile and in the fix_ssh() function.
• Improve ssh service name detection in fix_ssh()
• Implement some rubocop suggestions
• Remove unused sshd_options variable

[codecov]

Codecov Report

Base: 84.05% // Head: 84.74% // Increases project coverage by +0.69% 🎉

Coverage data is based on head (8cca573) compared to base (c35798f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   84.05%   84.74%   +0.69%     
==========================================
  Files           1        1              
  Lines         301      295       -6     
==========================================
- Hits          253      250       -3     
+ Misses         48       45       -3     

Impacted Files	Coverage Δ
lib/beaker/hypervisor/docker.rb	84.74% <100.00%> (+0.69%)	⬆️

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[jay7x]

I'd like to refactor a bit more here.. e.g. stop duplicating parts in dockerfile_for(), install_ssh_components(), fix_ssh(). So consider this as 1st PR of a series.