bin/bootstrap_toolchain.py should support URL environment variables for CDP components

To make it easy to override the Hadoop components used for building and developing Impala, it is useful to be able to specify custom versions/URLs for downloading components. These already exist for CDH through is use of IMPALA_*_URL environment variables. The same should be possible with CDP components.

This is also useful to allow trying out newer component packages before updating to a newer build number.