Promote libffi-dev as a general dependency

libffi-dev/libffi-devel can be requested for rebuilding the
python clients which is an un-skipable part of the devstack install.

Promoting the libffi-devel to a general dependency.

Change-Id: I1dac7fd536b94de1e49aaf335ac29ffae1bd39d1
Merge "Promote libffi-dev as a general dependency"