OCCI-OS contextualization support

* Wrapped helpers in Occi::Cli::Helpers (code clean-up)
* Added support for public_key and user_data contextualization for OCCI-OS

~~~
Category: user_data; scheme="http://schemas.openstack.org/compute/instance#"; class="mixin"
X-OCCI-Attribute: org.openstack.compute.user_data = "<BASE64 ENCODED DATA>"
~~~

~~~
Category: public_key; scheme="http://schemas.openstack.org/instance/credentials#"; class="mixin"
X-OCCI-Attribute: org.openstack.credentials.publickey.name="<YOUR_KEY_NAME>"
X-OCCI-Attribute: org.openstack.credentials.publickey.data="<YOUR_KEY_DATA>"
~~~