SPEC: have discovery walk up the tree

The basic idea is that people may not want to have a landing page for
every container name but will instead want a landing page for an overall
project. This would let people do this as the discovery algorithm will
walk up the tree from example.com/project/app1 to example.com/project to
example.com.
Merge pull request #52 from philips/issue_18

SPEC: have discovery walk up the tree
discovery: do not set defaults for os and arch.

os and arch can be optional as there can be "noarch" (without machine dependent
code) aci.
discovery: Ignore templates that cannot be completely rendered due to missing labels.

If meta discovery template cannot be completely rendered due to missing labels
it should not be added to the list of available download uris.

Also create the template vars slice from all the available labels instead of
only some predefined ones (name, version, os, arch).