fix(gcp): Avoid updating resource on second terraform apply

[afiune]

When you run terraform apply after creating the resource, we are trying to update the
private_key when it is sensitive and will always be different. We should avoid updating the
resource:

 # module.gcp_project_audit_log.lacework_integration_gcp_at.default will be updated in-place
  ~ resource "lacework_integration_gcp_at" "default" {
        created_or_updated_by   = "afiune@lacework.net"
        created_or_updated_time = "2020-Jul-16 15:34:02 UTC"
        enabled                 = true
        id                      = "TECHALLY_1234"
        intg_guid               = "TECHALLY_1234"
        name                    = "TF audit_log"
        org_level               = false
        resource_id             = "demo"
        resource_level          = "PROJECT"
        subscription            = "projects/demo/subscriptions/lw-at-demo-lacework-subscription"
        type_name               = "GCP_AT_SES"

      ~ credentials {
            client_email   = "lw-at-svc-account@demo.iam.gserviceaccount.com"
            client_id      = "1234"
          + private_key    = (sensitive value)
            private_key_id = "abcd"
        }
    }