Drop Plenary.plenary_directory
Drop Plenary.plenary_directory

Compute the value dynamically instead. This is preparation for when
Plenary.plenary_file will also be computed dynamically.

Change-Id: I595db274de50559d807c95019dee82239bf6e1d4
Reviewed-By: Priya Joshi <3c6cfa6f57440adfc837c4ad04483313acbc2582@morganstanley.com>

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