upstart support for controlling ElasticSearch

on Ubuntu, the default init daemon is upstart. We'd like to support it,
so here's a first attempt in that direction.

We're basing the general startup on Debian's sysV init script. By
extending the params to make sure the init_defaults already include
system-independent defaults, we can easily use those in our upstart
script.
By ensuring the defaults file contains "export" it enables us to
actually override variables in the upstart script.