Add generation of dropin.cache to make install target.
Add generation of dropin.cache to make install target.

The latest twisted complains loudly if there is no dropin.cache file
and it cannot write one.  This generates the file at install time and
prevents the harmless but large error from being generated on startup.

stdweird/aquilon,guillaume-philippon/aquilon,stdweird/aquilon,stdweird/aquilon,quattor/aquilon,quattor/aquilon,quattor/aquilon,guillaume-philippon/aquilon,guillaume-philippon/aquilon