Use the extensions style configuration for setting the SCM Git clean
configuration. With this format we deprecate the older (clean: bool)
configuration format and replace it with this format:
clean:
after: bool
before: bool
The older configuration is still supported so users who did not update
their YAML files can upgrade without updating all their YAML files first.
Change-Id: I1cfc0e2951c2b4de5e67bcb8abdf0b8c070e7fc6
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>