consumer - specify different unit key based on content type

When installing content (e.g. rpm, package_group, erratum)
using pulp, the 'unit key' used may be different based on
the content type.  This small commit addresses the changes
needed for the supported types.

e.g. rpm and package_group use a unit key of 'name',
     erratum uses a unit key of 'id'
Merge pull request #58 from bbuckingham/pulpv2-updates

consumer - specify different unit key based on content type