Fixed a bug where peer packages were not used

This occurred during rosdep key resolving, I had
added a generic interface for handling missing
rosdep keys, but I failed to completely
reimplement the functionality which was used
before.
Improved the pushing test
Resolved keys should be returned as a list of str
Merge pull request #160 from ros-infrastructure/issue_160

git-bloom-release: failing for freenect_stack