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.