Add package logic for Amazon Linux 2

[mhashizume]

This commit adds logic for Amazon Linux 2 to Beaker's install_ and uninstall_package methods. (Note: AL2 is referred to in Vanagon and ABS as Amazon Linux 7, in reference to the assumed compatibility with Red Hat Enterprise Linux 7.)

[joshcooper]

@bastelfreak could you review and merge?

[joshcooper]

I verified this works as expected:

$ export BEAKER_VERSION=https://github.com/mhashizume/beaker#maint/master/al-2
$ bundle install
$ env SHA=503cb74f9d28c4dbbc49343beab3e184bd8da10f HOSTS=amazon7-64a bundle exec rake ci:test:setup
...
  ip-10-227-0-206.amz-dev.puppet.net (ip-10-227-0-206.amz-dev.puppet.net) 21:02:30$ yum -y  install puppet-agent

@imaqsood could you add a test so we don't regress, similar to

beaker/spec/beaker/host/unix/pkg_spec.rb

Lines 156 to 162 in 7053494

	it "uses dnf on amazon-2023" do
	@opts = { 'platform' => "amazon-2023-is-me" }
	pkg = 'amazon_package'
	expect(Beaker::Command).to receive(:new).with("dnf -y install #{pkg}", [], { :prepend_cmds => nil, :cmdexe => false }).and_return('')
	expect(instance).to receive(:exec).with('', {}).and_return(generate_result("hello", { :exit_code => 0 }))
	expect(instance.install_package(pkg)).to eq "hello"
	end