Enable configurability of data storage

Currently Trove stores persistent data either on ephemeral volume, or an attached cinder volume. This is restrictive and it would be beneficial for Trove to be able to interface with some other kinds of abstractions. One example is to be able to store data on Manila based folders.

I am specifically looking into the benefits and implementation of an abstraction that would support Manila (https://wiki.openstack.org/wiki/Manila).

This blueprint is being registered to reflect the fact that this is an area that I'm investigating. I will file a trove-spec in the trove-specs repository once further investigation has been completed and I have enough information to both confirm that this is a good idea, and enough information to credibly describe the implementation.