(#3910) Make puppet agent use environment specified in catalog

If there is an environment specified in the catalog to be applied (i.e. an
external node classifier was used and gave an environment for this node) the
puppet agent will switch to that environment.

When switching environment, the agent needs to get facts and plugins again
using the new environment, and then try fetching the catalog again using the
new facts, since this may change the catalog. It loops in this process several
times, until the environment doesn't change. If it fails to stabilize in a
limited number of tries it will raise an error and abort the run.

This change also means that the prerun command runs after fetching catalog but before
applying. It previously ran before fetching the catalog.