Correctly set install prefix for 'bloom-generate rosdebian'

Currently the installation prefix is set to /usr for 'bloom-generate
rosdebian', even though the documentation states that it will be
/opt/ros/<rosdistro>. This also means that the debian packages can't be
built, as the pkgconfig path is wrong.
Merge pull request #288 from cberner/fix_prefix

Correctly set install prefix for 'bloom-generate rosdebian'