puppet: install ttf-dejavu package to fix java headless issue

Jenkins/Java with OpenJDK might complain about:

| "Caused by: java.lang.Error: Probable fatal error:No fonts found."

and e.g. the "Test Result Trend" graphics are missing then.

Might also require:

  JAVA_ARGS="-Djava.awt.headless=true"

inside /etc/default/jenkins. The latest Jenkins packages
already include this, but it's missing in the LTS releases
currently, jftr.