jenkins: fail fast during job execution

Jenkins will dynamically write the commands in its database into a
script and run it with "-x" and "-e" in order to display each step and
fail after the first step fails.

Now that our commands are stored in Git, ensure that we're doing the
same thing.

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>
Merge pull request #205 from kdreyer-inktank/jenkins

jenkins: fail fast during job execution