Openstack driver

Related to docker/machine#73:
- [x] Some `info` level should be at `debug` level or maybe not present at all
- [x] If tenant name and tenant id are both provided the driver should not fail
- [x] Call to `driver.AddPublicKeyToAuthorizedHosts()` is missing
- [x] Allow user to provide names or id instead only id for
   - [x] Flavor
   - [x] Image
   - [x] Network

Network name should not have a default value as long as there is no standard name. It's specific to each OpenStack installation.