Automatically accept host key

When i run `vagrant up --provider=openstack` i have the following error on ansible provisionning

```
TASK: [checkout devstack] *****************************************************
failed: [default] => {"failed": true}
msg: git.openstack.org has an unknown hostkey. Set accept_hostkey to True or manually add the hostkey prior to running the git module

FATAL: all hosts have already failed -- aborting
```

Adding the option `accept_hostkey=yes` on git module fix the bug.