(PA-1887) Fix osfamily::darwin OSX version regex

[ekinanp]

The previous regex used a character class instead of a union to
match the major Mac OSX product version, which would match values
such as "10.14", "10.15". This meant that the puppet_agent module
would think these are supported platforms, and proceed further with
the installation (failing at a different point, which would produce
a harder-to-read error message). This commit fixes things so that
non-supported Mac OSX platforms correctly fail in the osfamily::darwin
class with a readable error message.

[ekinanp]

A quick sanity run (https://cinext-jenkinsmaster-enterprise-prod-1.delivery.puppetlabs.net/job/experimental_test_playground_2017.3.2_2017.3.6-rc0-5-gcd13eb0_automate_run_pe_acceptance_tests/8/ and https://github.com/ekinanp/pe_acceptance_tests/commit/321af32f2bd4a426a93a926d2618c1f00af8427c for the r10k change) indicates that this PR does not break anything, so this should be good for a merge.

[puppetcla]

CLA signed by all contributors.

[MikaelSmith]

We should also update the spec test at https://github.com/puppetlabs/puppetlabs-puppet_agent/blob/master/spec/classes/puppet_agent_osfamily_darwin_spec.rb#L53

[ekinanp]

@MikaelSmith Oh forgot to tell you that I updated the spec test

[MikaelSmith]

This should probably have a ticket, since it will fix upgrades for 10.12 and 10.13. I assume those wouldn't work before.

[ekinanp]

What's interesting is those worked and still work fine -- the reason is b/c the old regex was wrong, it was using character classes. I talk about it a bit more in the commit message. I can file the ticket though.

[MikaelSmith]

Oh gotcha. Probably still worth ensuring we have a release note about now enforcing macOS version checks.

[ekinanp]

@MikaelSmith Ok, I filed PA-1887 for this work and it seems that the CI checks have passed.