Update changes in deb.rake for new library utils

The evaluation of spec files in deb.rake changed, and this wasn't updated when
the topic branch for the reorganization was rebased. This commit fixes its
usage to reference the new erb_file utility.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Add in the new bundle_platforms variable

The latest packaging repo in master supports 'bundle_platforms', but we don't
yet in this branch. This commit makes it so.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Add support for using reassignments to redirect initial variable values

In some cases, we're going to want to populate a variable with the value of
another one, e.g. project => gem_name. If gem_name isn't populated, we want to
use the value of project instead. This isn't going to work without some form of
assignment. This can be supplied by the Pkg::Config::issue_reassignments
method, which simply reassigns the value of an old variable to a new one, to
support all our old and crufty terrible ways.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>