Add certificate_name to load balancer data source.

[andrewsomething]

The load balancer data source and resource share a common flattenForwardingRules method:

terraform-provider-digitalocean/digitalocean/loadbalancer.go

Line 190 in a6eac97

func flattenForwardingRules(client *godo.Client, rules []godo.ForwardingRule) ([]map[string]interface{}, error) {

When using the load balancer data source with a load balancer that has a TLS certificate, we're seeing errors setting the data to state. This is because the shared method adds certificate_name to the forwarding rule, but currently only the resource uses that. This PR adds the attribute to the data source as well.

Fixes: #569

$ make testacc TESTARGS='-run=TestAccDataSourceDigitalOceanLoadBalancer_tlsCert'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccDataSourceDigitalOceanLoadBalancer_tlsCert -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDataSourceDigitalOceanLoadBalancer_tlsCert
=== PAUSE TestAccDataSourceDigitalOceanLoadBalancer_tlsCert
=== CONT  TestAccDataSourceDigitalOceanLoadBalancer_tlsCert
--- PASS: TestAccDataSourceDigitalOceanLoadBalancer_tlsCert (130.70s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean    130.714s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/mutexkv        (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/setutil        (cached) [no tests to run]

[bentranter]

LGTM!