bloom-release should ask the user for the release repository url if it is not in the rosdistro file

When releasing a package for the first time in a new distro (after the distro files have been forked) `bloom-release` will fail with something like this:

```
Specified repository '<repository_name>' is not in the distro file located at 'https://raw.github.com/ros/rosdistro/master/releases/hydro.yaml'
```

`bloom-release` then just exits.  It should instead request the location of the release repository from the user, using that url for the release process and adding that as an entry to the rosdistro file at the end.