Handle the hardcoded configuration values

This patch allows to customize the hardcoded configuration values,
as well as introduce new configuration options for designate services
by configmap template with the specified section:

  designate:
    server:
      configmap:
        DEFAULT:
          default_soa_refresh_min: 3500
          default_soa_refresh_max: 3600
          default_soa_minimum: 3600
        'network_api:neutron':
          admin_username: admin
          admin_password: password
          admin_tenant_name: tenant1

Change-Id: I43224d260bed265b21c748abe294c0a97bfb7f32
Related-Prod: PROD-29626
(cherry picked from commit 4e405f1414cdac9b628b1b92dce4d6fcf4d44786)
diff --git a/README.rst b/README.rst
index d43206c..2ed2f57 100644
--- a/README.rst
+++ b/README.rst
@@ -268,6 +268,23 @@
             name: test.zone.
             email: admin@test.zone
 
+Change default options using configmap template settings
+========================================================
+
+.. code-block:: yaml
+
+    designate:
+      server:
+        configmap:
+          DEFAULT:
+            default_soa_refresh_min: 3500
+            default_soa_refresh_max: 3600
+            default_soa_minimum: 3600
+          'network_api:neutron':
+            admin_username: admin
+            admin_password: password
+            admin_tenant_name: tenant1
+
 Upgrades
 ========