(QENG-1074) Override mac and win url for `install_puppet`

Previously we always downloaded windows or osx installation packages
from http://downloads.puppetlabs.com.

This allows the specifying of special urls for windows and osx to
`Beaker::DSL::InstallHelpers#install_puppet`.
Merge pull request #522 from justinstoller/allow_custom_download_urls

QENG-1074 Add options for mac and win download from alternative sources for install_puppet