Introduces two new mixins defined in the OCCI 1.2 spec for handling
contextualization: user_data and ssh_key. Functionality is similar to
the already existing OpenStack mixins, which may be deprecated in the
future.
Partially-Implements: blueprint occi-1-2-core
Change-Id: Ibb6624e3e15bf7b9f3ad39786dcefb36b1a9a797