(PP-295) Add PUPPET_AGENT_ENVIRONMENT command line option

Adds the ability to specify the environment configuration setting on
Windows during installation, for example:

    msiexec /qn /i puppet.msi PUPPET_AGENT_ENVIRONMENT=dev
Merge pull request #221 from joshcooper/issue/master/PP295-agent-environment

(PP-295) Add PUPPET_AGENT_ENVIRONMENT command line option