1149894 - Has tempdir selection for installdistributor use get_parent_directory from platform

https://bugzilla.redhat.com/show_bug.cgi?id=1149894

This is the pulp_puppet PR that corresponds with [PR 1192](https://github.com/pulp/pulp/pull/1192) against pulp/pulp. The other one should be merged first.

This change does not have unit tests because the entire installdistributor does not have unit tests and that is a bigger, different problem. I have filed the lack of unit tests as its own [BZ 1150136](https://bugzilla.redhat.com/show_bug.cgi?id=1150136).