Add two CI jobs that run e2e tests on GCE with Trusty images (on both the master
and the nodes) on the `master` and `release-1.2` branch. Both tests use the
latest Trusty build from its `HEAD`. We will use these jobs to guard
kubernetes-Trusty compatibility.

These jobs mimic the existing `kubernetes-e2e-gce` and `kubernetes-e2e-gce-1-2`,
which follow `kubernetes-build` and `kubernetes-build-1-2`, respectively.

Additionally, move all Trusty related jobs into one section in the config file.