EnterpriseGateway integration is broken (since 2.x)

[kevin-bates]

Describe the issue
The gateway configuration that comes from the server is not available when the server is using a gateway server. As a result, no local pipelines can be run against the gateway server. This has been broken since the move to Lab 3.0 (#1063).

To Reproduce
Steps to reproduce the behavior:

1. Using Elyra 2.x, start lab against a gateway server using --gateway-url
2. Confirm that notebooks using a gateway kernel work properly
3. Add one of those notebooks to a pipeline.
4. Attempt to run the pipeline locally will fail - indicating that the given kernel is not available.

Expected behavior
The notebook node in the local pipeline should behave just like it does outside of the pipeline (and when configured against a gateway server).

Deployment information
Describe what you've deployed and how:

• Elyra version: 2+
• Operating system: macos
• Installation source: from source
• Deployment type: local installation