CI: Add cleanWs to cleanup steps in jenkinsfiles

We're at the point where there are enough Jenkins jobs for
Asterisk branches than even cleaned checkouts of Asterisk
will add up to more disk space than is available on the
in-memory workspace mount.  Since we archive all relevent
artifacts anyway, there's no need to keep the workspace
around after the job finishes, whether it succeeds or fails.

Change-Id: I1cd3b73ebb045a987df0f62526d152a510210c39