Updates to RPM and DNF Package Manager

First commit makes `rosdep` try to use the python API for RPM when detecting packages. If it can't import it (which should never happen), it will fall back to using the current command-based method. The RPM python module is a C extension, and runs quite a bit faster than parsing command line output.

The second commit adds support for the upcoming DNF package manager in Fedora. Eventually we'll want to make this the default installer for F21+, but we'll cross that bridge when we come to it.