Retreive OpenStack specific metadata

Hi,

I have added for retrieve OpenStack specific metadata from metadata API .


[OpenStack specific metadata API](http://docs.openstack.org/grizzly/openstack-compute/admin/content/metadata-service.html)

The result of unit test is below.

```
OpenStack Plugin
  when there is no relevant hint
    does not set any openstack data
  when there is an `openstack` hint
    and the metadata service is not available
      does not set any openstack data
    and the metadata service is available
      reads the reservation_id from the metadata service
      reads the public_keys_0_openssh_key from the metadata service
      reads the security_groups from the metadata service
      reads the public_ipv4 from the metadata service
      reads the ami_manifest_path from the metadata service
      reads the instance_type from the metadata service
      reads the instance_id from the metadata service
      reads the local_ipv4 from the metadata service
      reads the ari_id from the metadata service
      reads the local_hostname from the metadata service
      reads the placement_availability_zone from the metadata service
      reads the ami_launch_index from the metadata service
      reads the public_hostname from the metadata service
      reads the hostname from the metadata service
      reads the ami_id from the metadata service
      reads the instance_action from the metadata service
      reads the aki_id from the metadata service
      reads the block_device_mapping_ami from the metadata service
      reads the block_device_mapping_root from the metadata service
      reads the provider from the metadata service

## Add this context
      Retreive openStack specific metadata
        reads the availability_zone from the openstack metadata service
        reads the hostname from the openstack metadata service
        reads the launch_index from the openstack metadata service
        reads the meta from the openstack metadata service
        reads the name from the openstack metadata service
        reads the public_keys from the openstack metadata service
        reads the uuid from the openstack metadata service

Finished in 0.16776 seconds (files took 0.12595 seconds to load)
29 examples, 0 failures
```
You can also make use of `http_client` here (https://github.com/opscode/ohai/blob/master/lib/ohai/mixin/ec2_metadata.rb#L97-L99)