Use dib-run-parts for running scripts in target

Also modified dib-run-parts to apply a more workable solution for
filtering out unwanted files such as editor backups and VCS.

The script is installed in its own element, depended on by the OS
specific ubuntu element. This is  because the ubuntu element (and
later other OS's) are responsible for populating the root filesystem.
If we try to install this in base, the root filesystem will look to
be populated already and we will skip automatically choosing ubuntu.

Change-Id: I017646748c1a8360299106289b57d976d45875a8