Asgard user data integration

[jhohertz]

More here: https://github.com/Netflix/asgard/wiki/Customizing-User-Data

Much of the default data Asgard injects maps closely to the environment config components expect in a non-test setting.

This means taking the variables from the user data Asgard sets, and applying them to the launch-time properties of underlying java application where applicable.

We can use the following construct in /etc/default/jetty:

-Dsome.var=$(shellcommand --params)

This at least gets us availabily of Asgard's environment injections at startup.

Needs more work yet, testing.