Currently if one wants to include a JRE it is needed to specify a
concrete EE for each of the linux/win/mac what is quite cumbersome and
could easily forgotten to update.

Instead this adds a new flag "includeJRE" in the product that enables
this for all os at once and always uses the default vm (what is the one
from the target platform or user chosen default).

This will also make it easier for Tools like Tycho that already
investigate the 'includeLaunchers' flag.