support executors in multiple AZ

[martin-paulus]

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Description

Support multiple profiles in the gitlab runner config (multiple [[runners]] sections), based on the subnets provided.

We required this because we saw our pipelines stalling due to unavailability of our preferred EC2 Spot machine type in our default availability zone. After extending the running confguration with more [[runners]] sections the launch failures were followed by successful launches in another availability zone.

Migrations required

YES - The value of var.subnet_id needs to be input (as a list) at var.executor_subnets to maintain backwards compatibilty.

Verification

(verification was based on a modified 4.18.0 release of the module, this exact PR is yet to be verified)

Documentation

Terraform inputs in the documentation were updated via terraform-docs.