Make api gateway optional and adjust api_base_auth var naming

[Erin-Boehmer]

Addresses issue #22

In production environments, some teams will not want to use public-facing API Gateway resources. This PR makes API Gateway optional by adding a var to the metadata service enable_api_gateway. I also adjusted the api_basic_auth var to use the enable_ prefix, in keeping with the bool var naming in this repo.

[oavdeev]

This change would technically require the module user to change the param name, right? Just making note of this so i'll make it a minor release then, not a patch release

[Erin-Boehmer]

This change would technically require the module user to change the param name, right? Just making note of this so i'll make it a minor release then, not a patch release

Hey @oavdeev - yes, this would be a minor release. I refactored the api_basic_auth to enable_api_basic_auth (within the metadata service module) and metadata_service_enable_api_basic_auth (at the metaflow module level). Also rebased against latest commits if you're ready to merge.