Add 'env' module for managing shell environments

This change adds a lightweight Puppet module for managing /etc/profile.d shell
environment initialization scripts. The module uses the same pattern as the one
used for managing MediaWiki configuration snippets, which is to recursively
manage a 'puppet-managed' subdirectory of an *.d tree.

Change-Id: Icc4ae6dcfa507327853c0bd4fe6a2e15df7e9a1e