openstack-full role on Debian must use kernel 3.12 from bpo for OVS 2.0.1 to work properly

Issue met with @EmilienM and discussed with @fredericlepied ; GRE tunnels are not mounted if this configuration is not met.

The trick is to install initramfs-tools from wheezy-backports prior to the 3.12 kernel. The role is building correctly in my eDeploy server, and this setup is working well in a Debian 7 platform for now.