Use Red Hat 7 for Amazon Linux 2 x86_64

[mhashizume]

In 4112690, I mapped Amazon Linux 2 (both x86_64 and AARCH64) to their respective ABS/Vanagon targets. This was incorrect, as AL2 x86_64 is still built on Red Hat Enterprise Linux 7.

This commit partially reverts that change and re-adds AL2 x86_64 to be built on RHEL 7.

[joshcooper]

It seems wrong to build on rhel7 to me, given we have amazon-7-x86_64 in the pool and it's different than how we handling ARM.

[mhashizume]

It seems wrong to build on rhel7 to me, given we have amazon-7-x86_64 in the pool and it's different than how we handling ARM.

@amitkarsale would it be feasible for your team to build on AL2 x86_64? Do we need this PR?

[nmburgan]

Note that we still need platform to be el-7-x86_64 so that we fetch the right PE tarball when we use amazon7 as a primary. We can probably work around it in beaker-pe if we need to, though.

[joshcooper]

Sorry I was conflating build and template. We build x86_64 packages on RHEL7 and install them on amazon7. This PR doesn't change which platform we build on. But we do want to request an amazon-7-x86_64 template (which this PR restores).

[joshcooper]

$ export BEAKER_HOST_GENERATOR_VERSION=https://github.com/mhashizume/beaker-hostgenerator#maint/master/al2-x86 
$ cd ~/work/facter/acceptance
$ bundle install
...
$ env SHA=503cb74f9d28c4dbbc49343beab3e184bd8da10f HOSTS=amazon7-64a bundle exec rake ci:test:setup 
...
$ ssh ip-XXX.amz-dev.puppet.net
Warning: Permanently added 'ip-XX.amz-dev.puppet.net,XXX' (ECDSA) to the list of known hosts.

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       _|  (     /   Amazon Linux 2 AMI
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