Add tools/make_cert.sh

This allows use of either the DevStack CA or creating another CA
independent of stack.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