953665 - added the ability for copy operations to not also copy child un...

...its, such as a group copying its RPMs. Also restricted the fetching of existing units to their unit key fields, which reduced RAM use tremendously. Copying a RHEL6 repo went from using about 4.3GB of RAM to < 100MB.

https://bugzilla.redhat.com/show_bug.cgi?id=953665

Please review with: https://github.com/pulp/pulp/pull/443