Allow instance types to be pulled from github.com

[Fryguy]

I was thinking about how instance types are not updated until the next refresh and once we've merged #752, I wonder if those values can be pulled live from this repo during refresh, or perhaps on demand. I haven't worked out all of the details in my head, but some points I'm thinking:

1. We would need to include a version number or flag or something in the yaml file and/or permanently ensure backward compatibility.
2. Since it's unlikely to update that frequently we might want to use an ETag to avoid pulling something that hasn't changed.
3. Failure to fetch, update, anything would gracefully write a log message and move on.
4. Users should be able to "opt-out" or "opt-in" in case they are deploying into a DMZ and don't want to be pulling from github.com

[Fryguy]

some info on the etag:

$ curl -i https://raw.githubusercontent.com/ManageIQ/manageiq-providers-amazon/master/db/fixtures/aws_instance_types.yml
HTTP/2 200
...
etag: "f45f89c670445b64ace7a519c008c904c7d267def483f665487b0f60b1719734"
...

[miq-bot]

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