puppet-agent upgrade should specify Windows MSI REINSTALLMODE

Windows installer behavior may corrupt Puppet installations when downgrading a newer puppet-agent version to an older puppet-agent version, by removing files that the application needs.

One strategy for dealing with this problem is to provide additional arguments to msiexec.exe during installation to force files always get copied. The agent upgrade module already supports arbitrary argument passing as part of MODULES-4214 / https://github.com/puppetlabs/puppetlabs-puppet_agent/commit/458a747ba58c80136125eadea7a043cfc7b7ca4e

PA-1522 covers verifying that the new property value works (and setting it by default in the MSI). Given installers have already shipped and cannot be fixed, we must document this workaround for end users.

DOCUMENT-724 covers adding this to our installation docs.

This ticket is about either:

• Adding a note to the README as a known issue for anyone wishing to downgrade, and how to use the existing support from MODULES-4214 to work around the problem
• Automatically supplying the REINSTALLMODE value so that a user doesn't have to do this, as we're aware this is a universal problem - the hitch there is dealing with a situation where a user wants to override our value