(SERVER-1995) Check env vars for branch config in submodule rake task
(SERVER-1995) Check env vars for branch config in submodule rake task

This commit updates the puppet submodule bump rake task to check
environment variable for the server and puppet branches before falling
back to hard coded values. These env vars can be set either by the user
on the command line or in Jenkins via job params.

puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver