ec2: ebs_optimized can be optional

I have old instance of OpenStack and with ansible 1.6 I got traceback from ec2 module:

```
failed: [localhost] => {"failed": true, "parsed": false}
invalid output was: Traceback (most recent call last):
  File "/home/copr/.ansible/tmp/ansible-tmp-1401461599.43-107043805508227/ec2", line 2539, in <module>
    main()
  File "/home/copr/.ansible/tmp/ansible-tmp-1401461599.43-107043805508227/ec2", line 1170, in main
    (instance_dict_array, new_instance_ids, changed) = create_instances(module, ec2)
  File "/home/copr/.ansible/tmp/ansible-tmp-1401461599.43-107043805508227/ec2", line 956, in create_instances
    d = get_instance_info(inst)
  File "/home/copr/.ansible/tmp/ansible-tmp-1401461599.43-107043805508227/ec2", line 574, in get_instance_info
    'ebs_optimized': inst.ebs_optimized}
AttributeError: 'Instance' object has no attribute 'ebs_optimized'
```
This does not happened with ansible 1.4.

It seems that ```ebs_optimized``` is not always returned and ansible should accomodate.