Mount automation framework for Manila

We need to implement multiple options for mount automation so that users can choose the one that works best for them or even create their own. This requires a pluggable framework for adding mount automation functionality.

Mount automation can be implemented in a number of different ways (push, pull, publish/subscribe) with wildly different technologies (SSH, Cloud-init, agent with zeroconf or Zaqar notifications, AutoFS+LDAP, etc.). No one solution will work for all deployments, so Manila should provide a framework that allows multiple options to be implemented over time.

Design draft can be found here: https://docs.google.com/document/d/1Z-VIADEBNrwvuIuymcER8j4d5YcQHy49H_pV0IukYK8/edit?usp=sharing