Add a seperate store for MasterTemplate

Create a new store for the Master Templates as they don't need to be
versioned and add helper methods to the Deployment Plan store to make it
easier to update.

These changes are based on feedback from usage of the store API.

Change-Id: I3dca6f09e5e802ff5cdc454d4f0d389fbdb50b3b
Implements: blueprint tripleo-juno-tuskar-template-storage
Merge "Add a seperate store for MasterTemplate"