Have a default path for quantum-rootwrap

Finding the path to quantum-rootwrap in a ruby_block makes
node[:quantum][:rootwrap] exist too late to be used when populating
quantum.conf in the first chef-client run. This results in some failures
to use sudo, which are not critical but annoying.

Instead of proceeding this way we just default the path to
quantum-rootwrap to /usr/bin/quantum-rootwrap (correct path for
upstream, Ubuntu and SUSE), and keep the ruby_block to fix the path if
it's not the same. Things will still fail in the first chef-client run
if it's not the right path, but chef convergence will make it work for
later runs. And it now works directly in the first run in most cases.
Merge pull request #57 from vuntz/fix-finding-rootwrap

Have a default path for quantum-rootwrap