Add mechanism to automatically load additional projects

This adds an extras.d directory to contain startup scripts that
stack.sh runs automatically at the end.  Similar to local.sh
except the scripts are sourced into the stack.sh process rather
than executed as a child process.  This gives them complete
access to the stack.sh environment.

Change-Id: Ibc95e6aaecf4211da948319eb452293ae4357780
Add tools/make_cert.sh

Change-Id: I055679b5fd06e830c8e6d7d7331c52dd8782d0b6
Merge branch 'fix-tempest-flavors' into dt-test
Merge branch 'tls-split' into dt-test
Merge branch 'extras-dir' into dt-test