Make Nova V3 API optional and disabled by default

It seems upstream decided to put v3 off by default for
icehouse. So adjust our cookbook accordingly.
Merge pull request #375 from dirkmueller/master

Make Nova V3 API optional and disabled by default
Synchronize nova.conf template with Icehouse

It's a huge patch, but with no functional change.
Set control_exchange to nova in nova.conf

This is what ceilometer expects (instead of the openstack value).