(PUP-1596) Read environment.conf for new directory environments

Prior to this commit, the new directory environments would always have a
set manifest and modules directory relative to their environment
directory.  This is too restrictive compared with the arbitrary
modulepath and manifest settings available to legacy puppet.conf
environments, so we are adding a per directory environment.conf which
allows arbitrary settings for manifest, modulepath and config_version
per directory environment.

The path to the environment.conf is unique per directory environment and
is not configurable.  For a given $environment found through the
$environmentpath, it is always
"<path-in-environmentpath>/$environment/environment.conf"

No sections are allowed in this file, and the only three settings
allowed are manifest, modulepath and config_version.  Adding a section
or other setting will log an error and cause the environment to be
ignored.  A relative path is always taken to be relative to the
environment directory itself.