Enable mock postinstall feature in RPM builds (#805)
Enable mock postinstall feature in RPM builds (#805)

This flag causes mock to attempt to install the packages it just built
after the build succeeds. I left this feature out initially because
there was a bug that caused the `--no-cleanup-after` flag to be ignored
when it is used, but the fix was merged and released upstream.
ros-infrastructure/ros_buildfarm,ros-infrastructure/ros_buildfarm,ros-infrastructure/ros_buildfarm