Fix output of apigee-x-core and apigee invocation

[V0idC0de]

What's changed, or what was fixed?

• output apigee-x-core.instance_endpoints is supposed to be a map of instance name => instance endpoint IP, which is not the case. When printed, the output turns out to map region => instance endpoint IP. Changed this to properly reflect the instances actual name.
• in apigee-x-core's locals is a definition for local.instances, which doesn't pass the key (desired instance name) from the instance-map set as variable into local.instances. This local variable is then passed to module apigee, which falls back to a default-name, due to the lack of a name field. This causes a de-sync between the expected instance name and the actual instance name.

My environment successfully built up after applying the changes, as looking up the expected instance name in the apigee-x-core.instance_enpoints map no longer causes Terraform's each.key doesn't reference an object in this map error. Printing the output values confirms, that they are now according to documentation (output description and README.md documentation).

Fixes: #137

• [x] I have run all the tests locally and they all pass.
• [X] I have followed the relevant style guide for my changes.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.