Switch from mixed case to uppercase env vars

[bastelfreak]

No description provided.

[ekohl]

I think you missed some:

$ rg 'BEAKER_[a-z]'
spec/spec_helper_acceptance_spec.rb
6:      before(:example) { ENV['BEAKER_provision'] = 'no' }

README.md
39:BEAKER_setfile=centos7-64 bundle exec rake beaker

[ekohl]

Note to self: I checked that it was a feature in beaker-rspec 8.0.0 and that's the minimum version in the gemspec.

[ekohl]

@evgeni pointed out this can be considered breaking. Let's fix it up and check for both.