Added chefspec tests to horizon cookbook

Have not added template tests at this time, but covered
the basic unit tests.  Opscode will probably not want this
upstreamed, but we need test, and test-kitchen cannot
handle search and such.

These are unit-tests anyways, and IMO perfectly acceptable.
cleaned up spacing and formatting

Formatted similar to our other openstack cookbooks.
Updated metadata.rb with supported OSes.
Corrected spacing (this isn't python)
We don't need a rackspace specific code
Removed a work-a-round that was fixed in essex