Add e2e test for HTTP cert SAN update and verification

[thbkrkr]

Add an e2e test to cover SAN usage and cert verification in 5 steps:

• Create an ES cluster with a load balancer HTTP service
• Retrieve the ES CA certificate
• Eventually retrieve the ES load balancer public IP
• Check that ES is not reachable with cert verification (error contains x509: cannot validate certificate)
• Add the load balancer IP to the SAN and update the ES definition
• Check that ES is reachable with cert verification (and returns 401)

    --- PASS: TestUpdateHTTPCertSAN (238.56s)
    --- PASS: TestUpdateHTTPCertSAN/K8S_should_be_accessible (0.17s)
    --- PASS: TestUpdateHTTPCertSAN/Elasticsearch_CRDs_should_exist (0.07s)
    --- PASS: TestUpdateHTTPCertSAN/Remove_Elasticsearch_if_it_already_exists (0.17s)
    --- PASS: TestUpdateHTTPCertSAN/Creating_an_Elasticsearch_cluster_should_succeed (0.08s)
    --- PASS: TestUpdateHTTPCertSAN/Elasticsearch_cluster_should_be_created (0.05s)
    --- PASS: TestUpdateHTTPCertSAN/ES_certificate_authority_should_be_set_and_deployed (3.18s)
    --- PASS: TestUpdateHTTPCertSAN/ES_version_should_be_the_expected_one (0.06s)
    --- PASS: TestUpdateHTTPCertSAN/ES_pods_should_eventually_be_running (150.88s)
    --- PASS: TestUpdateHTTPCertSAN/ES_services_should_be_created (0.06s)
    --- PASS: TestUpdateHTTPCertSAN/ES_pods_should_eventually_be_ready (12.33s)
    --- PASS: TestUpdateHTTPCertSAN/ES_pods_should_eventually_have_a_certificate (0.12s)
    --- PASS: TestUpdateHTTPCertSAN/ES_services_should_have_endpoints (0.06s)
    --- PASS: TestUpdateHTTPCertSAN/ES_cluster_health_should_eventually_be_green (9.20s)
    --- PASS: TestUpdateHTTPCertSAN/ES_cluster_UUID_should_eventually_appear_in_the_ES_status (0.06s)
    --- PASS: TestUpdateHTTPCertSAN/Elastic_password_should_be_available (0.06s)
    --- PASS: TestUpdateHTTPCertSAN/Elasticsearch_data_volumes_should_be_of_the_specified_type (0.07s)
    --- PASS: TestUpdateHTTPCertSAN/Every_secret_should_be_set_so_that_we_can_build_an_ES_client (0.11s)
    --- PASS: TestUpdateHTTPCertSAN/ES_cluster_health_endpoint_should_eventually_be_reachable (2.89s)
    --- PASS: TestUpdateHTTPCertSAN/ES_version_should_be_the_expected_one#01 (0.06s)
    --- PASS: TestUpdateHTTPCertSAN/ES_endpoint_should_eventually_be_reachable (0.06s)
    --- PASS: TestUpdateHTTPCertSAN/ES_nodes_topology_should_eventually_be_the_expected_one (0.13s)
    --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
    --- PASS: TestUpdateHTTPCertSAN/Retrieve_ES_certificate (0.10s)
    --- PASS: TestUpdateHTTPCertSAN/Retrieve_ES_public_IP (0.05s)
    --- PASS: TestUpdateHTTPCertSAN/Check_ES_is_not_reachable_with_cert_verification (0.12s)
    --- PASS: TestUpdateHTTPCertSAN/Add_load_balancer_IP_to_the_SAN (0.12s)
    --- PASS: TestUpdateHTTPCertSAN/Check_ES_is_reachable_with_cert_verification (50.07s)
    --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
    --- PASS: TestUpdateHTTPCertSAN/Deleting_Elasticsearch_should_return_no_error (0.07s)
    --- PASS: TestUpdateHTTPCertSAN/Elasticsearch_should_not_be_there_anymore (3.11s)
    --- PASS: TestUpdateHTTPCertSAN/Elasticsearch_pods_should_be_eventually_be_removed (3.12s)
PASS
ok      github.com/elastic/cloud-on-k8s/operators/test/e2e/es   238.572s

Resolves #724.

[sebgl]

LGTM