Refactoring repo urls.

Modifying how repo urls are computed and used by the recipes to allow
easy overloading of urls for specific components. This is mostly
useful for managing installs from various internal sources for
testing.
Remove the default for Openstack version attribute.

There exists no reasonable defaults for the openstack version
attribute and defaulting to the wrong version can result in a
seemingly working installation that's actually subtly broken. Instead,
we now provide no default values and raise an error if no version is
given.
Merge pull request #12 from rmahmood/master

Fix incorrect package ordering in compute recipe.
Merge pull request #11 from rmahmood/independent-urls

Refactor repo urls to allow specification of independent urls for each component.
Fix apt sources file naming.

Fix the sources files names so that we don't end up with multiple
sources files for the same repository. In retrospect, it never really
made sense to name the sources files after recipes.