[terraform-ibm-icd-edb] Add workaround to ignore groups config when provisioning instance using backup CRN

[ocofaigh]

There is a bug in the provider / service that we need to add a workaround for. We need to not pass any config for groups when a value is passed for the backup_crn input. Take same approach as was done in main.tf of this PR. (that PR is not complete - wait until fix for terraform-ibm-modules/terraform-ibm-icd-postgresql#515 is in too)

update - 18th Nov: there was one more fix needed in this area for PITR: terraform-ibm-modules/terraform-ibm-icd-postgresql#525