fix ROS installation prefix for RPM generator (#741)

The default 'installation_prefix' is '/usr'. This is wrong for 'rosrpm'
and has to be prefixed correctly with the default ROS installation.

Author: christianrauch

bloom/generators/rosrpm.py

@@ -163,7 +163,8 @@ def get_subs(pkg, os_name, os_version, ros_distro):
         pkg,
         os_name,
         os_version,
-        ros_distro
+        ros_distro,
+        RosRpmGenerator.default_install_prefix + ros_distro,
     )
     subs['Package'] = rosify_package_name(subs['Package'], ros_distro)
     return subs