(PUP-1118) Add check for getting multiple environments

The acceptance test now checks for getting multiple environments listed
from the environmentsdir
(PUP-1118) List environments from a directory

This starts the process of creating a Puppet::Environments::Directory
that will find environments from a particular directory. The directory
has sub-directories, where each sub-directory name is the environment
name.
(PUP-1118) Load full environments from a directory

This changes the loader used by the code to list environments to be a
composition of the legacy environments and the new directory
environments.