bloom should allow an alternative "push" url for the repository

The rosdistro file should always reference the https version of the url so that the build farm can always clone it (the ssh url can only be cloned if the cloner can authenticate with an ssh key).

This is a problem for users who do not use a ~/.netrc file as it prompts them at each push (three per release). Therefore bloom should provide an option in the release repository which allows the user to set a custom push url which is potentially different from the repository url in the rosdistro file.

Related issues:

#136 
#132 

https://github.com/ros/rosdistro/pull/676