Users may not want to build images with a cache as it significantly
slows down the build process and results in far larger images. Currently
we don't depend on cache-devstack in the jenkins-slave element for this
reason. Unfortunately we assume cache-devstack was run because we
attempt to link to a directory created by this element.
Update jenkins-slave to only make the link if the cache dir exists to
address this. Users can now make cacheless images quickly and cheaply.
Change-Id: I45ec2fcd614d71f1db85cab3c5c3730a96c8fd61
86877df4af