rax: import novaclient.exceptions for cs.images.find

`cs.images.find(human_id=` throws novaclient.exceptions.NotFound, resulting in the try/except block with
`image = cs.images.find(name=image)`  being skipped. catching novaclient.exception.NotFound allows  images to be specified with the human readable name.

Example:
```
  tasks:
    - name: Server build request
      local_action:
        module: rax
        region: DFW
        image: Ubuntu 12.04 LTS (Precise Pangolin)
```
Also, the import is placed after try: import pyrax, because pyrax
imports novaclient and should fail if novaclient is missing.