Add hooks to stack.sh, unstack.sh and clean.sh to call the extras.d
scripts at multiple points in stack.sh. This allows these scripts to
perform installation and startup tasks at similar times as they would
if integrated into stack.sh.
extras.d/70-tempest.sh is present as an example of the structure
of these scripts.
See extras.d/README.md for more information.
Change-Id: Ic1fe522559b94d204d6c0319a2e3d23684c8d028