Running Cube with pre-aggregations disabled

**Is your feature request related to a problem? Please describe.**
I have a complex Cube project with an external pre-aggregation database and pre-aggregations set up. However, I also wanted to run it locally without pre-aggregations, just to test a few schema changes. In order to do so, I needed to remove the pre-aggregations from my schema files.

**Describe the solution you'd like**
I'd like to be able to run Cube.js with a configuration option that makes Cube.js ignore pre-aggregations and always hit the raw data, e.g.,`CUBEJS_PRE_AGGREGATIONS=false`.

**Describe alternatives you've considered**
Using dynamic schema generation to wrap my cubes and remove `preAggregations` from them based on a custom configuration flag.

Start with Cube.js configuration handling and the schema files' preAggregations settings; compare the normal and dynamic schema-generation paths. Verify how a proposed CUBEJS_PRE_AGGREGATIONS=false setting would make local queries use raw data while leaving existing schemas intact, including projects using an external pre-aggregation database.

Written by the indexing model from the issue text.