Jenkins step to pre-clean workspace

Cannot use JJB plugin-in to clean-up workspace, because there will
be files owned by multiple user IDs.

Initially only adding to node-test job in order that it can be verified
and the new scripts can propagate to all nodes before adding it to other
job types.

Change-Id: Ibfe049355336a3d2bef65f681eab674a374fa45e
Partial-Bug: 1403188