Let's just rely on our Makefile everywhere. Plus the previous version
was essentially broken as _PULL_BASE_REF env variable was not available
in testing jobs.
Add a new script for running end-to-end tests. Implement a Makefile rule
for polling the container image. We need this for prow as there is no
other way for syncing the jobs (e2e postsubmit job must wait until the
image-pushing job has built and published the image). This is
implemented in the Makefile as all the image-related variables are also
defined/available there.
Also, adapt the TravisCI config to use the new script.
