use {obj:key} as a way to pass an object and not only strings to templates

As mentioned at the mail list[0]:

"So I need another way how to pass a *list* as data structure from
the defaults/project down to the job-template definition, similar to
what can be done with "{variable}" for strings."

[0] http://lists.openstack.org/pipermail/openstack-infra/2014-February/000799.html

Closes-Bug: 1281038
Change-Id: I5b04ddb52e537fedd2d05795e91011afd92f2295
Merge "use {obj:key} as a way to pass an object and not only strings to templates"