Add easy switching from DBs to in-memory

[jakerella]

I would like to have a very simple switch (not sure how exactly) to be able to switch all models to use the in-memory datasource versus a proper DB. This would make it much easier to demo the project.

[Sequoia]

model-config.DEMO.json?

[jakerella]

You have to change datasources.json as well since LoopBack checks DB connections at startup.

[Sequoia]

datasources.DEMO.json?