fix(osmap.yaml): fix PGDG repo file creation for Amazon Linux 2

[Niyakiy]

Fix improper pgdg repo file creation for Amazon Linux 2.

Description:
Current formula implementation uses
baseurl: 'https://download.postgresql.org/pub/repos/yum/{{ repo.version }}/redhat/rhel-$releasever-$basearch'
setting $releasever to 2 for Amazon Linux 2 and making formula install fail due to missing rhel-2 packages.

The fix makes a newly created formula repo file to use rhel-7 as the proper repository to use during install.

[myii]

Thanks for the PR, @Niyakiy. Please see the requested change below.

[myii]

@Niyakiy This if block can be avoided by adding this change to osmap.yaml instead, in a similar manner to this block:

postgres-formula/postgres/osmap.yaml

Lines 6 to 9 in 43c5696

	Fedora:
	pkg_repo:
	# yamllint disable-line rule:line-length
	baseurl: 'https://download.postgresql.org/pub/repos/yum/{{ repo.version }}/fedora/fedora-$releasever-$basearch'

I.e. Probably:

Amazon:
  pkg_repo:
    baseurl: 'https://download.postgresql.org/pub/repos/yum/{{ repo.version }}/redhat/rhel-7-$basearch'

This isn't perfect, since it doesn't specify the version of Amazon Linux but it is probably sufficient, since we're no longer supporting Amazon Linux 1 (2018). We could use the oscodename grain (Amazon Linux 2) in codenamemap.yaml but that's overkill in my opinion. Ultimately, we have a new map.jinja that is being introduced across formulas and we can provide the best solution when that becomes available.

[Niyakiy]

@myii Thanks for the heads up on a better and easier way to introduce a fix. Having AL1 already deprecated it looks reasonable to leave Amazon inside osmap.yaml as we have only the 2nd version available. However, it will require additional code changes here once AWS releases a new major version of AL.
I'm about to push the requested code changes along with reworded commit message.

[myii]

Just another note, doesn't necessarily have to be addressed by this PR.

---

We're not currently testing use_upstream_repo for most platforms, so this PR isn't actually being tested properly. Need to consider updating this block in the test pillar:

postgres-formula/test/salt/pillar/postgres.sls

Lines 10 to 18 in d16e91c

	postgres:
	# UPSTREAM REPO
	# Set True to configure upstream postgresql.org repository for YUM/APT/ZYPP
	{%- if grains.os_family != 'Debian' and salt['grains.get']('osfinger') != 'CentOS-6' %}
	use_upstream_repo: False
	{%- else %}
	use_upstream_repo: True
	# Version to install from upstream repository (if upstream_repo: True)
	version: '13'

I'll try to run some tests when I get a chance. Let's not make this a blocker for this PR, if I don't report back soon.

@Niyakiy Please don't go out of your way to run these tests, what you've already provided is more than enough, thanks.

[myii]

Thanks, @Niyakiy -- merged.

Out of interest, did you need to adjust the pkg_deps to also install these packages?

  pkgs_deps:
    - libicu
    - systemd-sysv

I had to do that to get PostgreSQL 13 installed on Amazon Linux 2:

• https://gitlab.com/myii/postgres-formula/-/jobs/1402800380

[saltstack-formulas-travis]

🎉 This PR is included in version 0.41.9 🎉

The release is available on GitHub release

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[myii]

@Niyakiy I've refactored this Amazon Linux 2 support in #315 -- feel free to comment on it there.