Update cluster could check if specified AMI ID is valid

[a1exsh]

Currently it will happily terminate the node to find out later that it cannot start a replacement:

  File "/home/oshulgin/src/planb-cassandra/planb/update_cluster.py", line 202, in build_run_instances_params
    image = ec2.describe_images(ImageIds=[params['ImageId']])['Images'][0]
  File "/home/oshulgin/.local/lib/python3.5/site-packages/botocore/client.py", line 253, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/home/oshulgin/.local/lib/python3.5/site-packages/botocore/client.py", line 557, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (InvalidAMIID.NotFound) when calling the DescribeImages operation: The image id '[ami-0123xxx]' does not exist

Probably we should also fail early if docker image we try to update with cannot be found.

[a1exsh]

Duplicate of #183