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.

Convert Tempest to use this format as an example.

Change-Id: Ibc95e6aaecf4211da948319eb452293ae4357780
Merge branch 'fix-tempest-flavors' into dt-test
Merge branch 'tls-split' into dt-test
Merge branch 'extras-dir' into dt-test