puppet/apply.sh: limit amount of retries for wget-ing EC2 info to 3

Otherwise wget retries retrieval of the file for up to 20 times.
jenkins_debian_glue.pp: depend on default-jre-headless package

Jenkins depends on a working Java runtime package but has just
'java2-runtime' in its depends (which is a virtual package),
this results in a broken default installation. So make sure
a Java runtime package is picked up by explicitely installing
default-jre-headless.

Thanks: Cyril Brulebois for the bug report