Puppet agent may download all plugins after updating

When using a server-specified environment, the agent will re-download all of its plugins after updating to 6.25.0 or 7.12.0. This occurs, because the agent cannot determine what the last server-specified environment was. So it will:

• fallback to "production"
• download plugins from "production"
• request a catalog from "production"
• be redirected to the original environment
• download plugins from the original environment
• request a catalog from the original environment

If the puppet-agent package on the server is not updated to 6.25/7.12 or later, and the server-specified environment references a fact that doesn't exist in the "production" environment, then the "hop back" to the original environment will fail as described in PUP-9570.

To prevent the agent from redownloading all plugins, the agent should fallback to making a node request if its last run summary does not contain the last server-specified environment.

This was originally reported in PUP-11323 and PUP-11327.