[terraform-ibm-observability-da] Add a test to test existing resources

[ocofaigh]

Add a test to test the following 2 use cases:

• using existing COS and COS buckets
• using existing COS and KMS key (so the DA creates new COS bucket and encrypts them with an existing KMS key)

Take the same approach that was taken in the SCC DA test -> https://github.com/terraform-ibm-modules/terraform-ibm-scc-da/blob/be22e36f2ad23a26af718d4a34b65f9b821a6ca7/tests/pr_test.go#L112

[kierramarie]

For the first test is it supposed to be one bucket or multiple buckets for one or all the services (e.g. log archive, and activity tracker)?

[kierramarie]

Test has been implemented and is in the process of being tested and debugged.

[kierramarie]

Found a bug in Instances solution where the bucket_config_map is being set to null when existing resources are used, however its length is checked several times causing errors. A fix will be addressed in the PR.

[kierramarie]

PR has been merged. Closing issue.