Allow Ansible installs without any target namespace

When installed in Kubernetes clusters v1.12 or later, target namespaces
can be configured at any time.

Touching only one namespace during install is a requirement of the GCP
Marketplace Kubernetes application installer.

Issue: [ch6131]

Author: cbandy

ansible/roles/pgo-operator/defaults/main.yml

@@ -29,6 +29,8 @@ pgo_tls_no_verify: "false"
 pgo_disable_eventing: "false"
 pgo_apiserver_port: 8443
 
+namespace: ""
+
 delete_operator_namespace: "false"
 delete_watched_namespaces: "false"
 delete_metrics_namespace: "false"

ansible/roles/pgo-operator/tasks/namespace.yml

@@ -6,6 +6,7 @@
     operator_namespace: !unsafe "{{.OperatorNamespace}}"
     target_ns_script: "{{ lookup('template', 'add-targeted-namespace.sh.j2') }}"
   with_items: "{{ namespace.split(',') }}"
+  when: namespace != ''
   tags:
   - install
   - update

ansible/roles/pgo-preflight/tasks/check_inventory.yml

@@ -8,7 +8,6 @@
   vars:
     mandatory_inventory_vars:
     - pgo_operator_namespace
-    - namespace
     - pgo_installation_name
     - pgo_admin_username
     - pgo_admin_password

hugo/content/Installation/install-with-ansible/prerequisites.md

@@ -131,7 +131,6 @@ sets of variables cannot be used at the same time.
 * `exporterport`
 * `kubernetes_context` (Comment out if deploying to am OpenShift environment)
 * `metrics`
-* `namespace`
 * `openshift_host` (Comment out if deploying to a Kubernetes environment)
 * `openshift_password` (Comment out if deploying to a Kubernetes environment)
 * `openshift_skip_tls_verify` (Comment out if deploying to a Kubernetes environment)
@@ -194,7 +193,7 @@ sets of variables cannot be used at the same time.
 | `log_statement`                   | none        |          | Set to `none`, `ddl`, `mod`, or `all` to configure the statements that will be logged in PostgreSQL's logs on all newly created clusters.                                        |
 | `metrics`                         | false       | **Required** | Set to true enable performance metrics on all newly created clusters.  This can be disabled by the client.                                                                       |
 | `metrics_namespace`               | metrics     |          | Configures the target namespace when deploying Grafana and/or Prometheus                                                                                                         |
-| `namespace`                       |             | **Required** | Set to a comma delimited string of all the namespaces Operator will manage.                                                                                                      |
+| `namespace`                       |             |          | Set to a comma delimited string of all the namespaces Operator will manage.                                                                                                      |
 | `openshift_host`                  |             | **Required**, if deploying to OpenShift | When deploying to OpenShift, set to configure the hostname of the OpenShift cluster to connect to.                                                                               |
 | `openshift_password`              |             | **Required**, if deploying to OpenShift | When deploying to OpenShift, set to configure the password used for login.                                                                                                       |
 | `openshift_skip_tls_verify`       |             | **Required**, if deploying to OpenShift | When deploying to Openshift, set to ignore the integrity of TLS certificates for the OpenShift cluster.                                                                          |