Add pooling support for Amphorae

Add configuration option and associated template updates to
support pooling of spare Amphorae to reduce load balancer
service creation time.

Change-Id: Idb49b632c5728c2bbace4124897fde7369cb803e

src/config.yaml

@@ -112,3 +112,11 @@ options:
     description: |
       Restrict glance image selection to a specific owner ID. This is a
       recommended security setting.
+  spare-pool-size:
+    type: int
+    default:
+    description: |
+      Number of Amphora instances to hold in the spare pool to reduce spin-up
+      time for new load balancers.
+      .
+      The default behaviour is to not maintain any spare servers.

src/lib/charm/openstack/octavia.py

@@ -276,6 +276,22 @@ def amp_flavor_id(cls):
         leadership.leader_get('amp-flavor-id'))
 
 
+@charms_openstack.adapters.config_property
+def spare_amphora_pool_size(cls):
+    """Number of spare Amphora instances to pool
+
+    Octavia can maintain a pool of Amphora instance to reduce the spin up
+    time for new loadbalancer services.
+
+    :param cls: charms_openstack.adapters.ConfigurationAdapter derived class
+                instance.  Charm class instance is at cls.charm_instance.
+    :type: cls: charms_openstack.adapters.ConfiguartionAdapter
+    :returns: Number of amphora instances to pool.
+    :rtype: str
+    """
+    return ch_core.hookenv.config('spare-pool-size')
+
+
 class OctaviaCharm(charms_openstack.charm.HAOpenStackCharm):
     """Charm class for the Octavia charm."""
     # layer-openstack-api uses service_type as service name in endpoint catalog

src/templates/rocky/octavia.conf

@@ -10,6 +10,10 @@ controller_ip_port_list = {{ options.controller_ip_port_list }}
 {% endif -%}
 heartbeat_key = {{ options.heartbeat_key }}
 
+[house_keeping]
+{% if options.spare_amphora_pool_size -%}
+spare_amphora_pool_size = {{ options.spare_amphora_pool_size }}
+{% endif %}
 
 [controller_worker]
 {% if options.amp_image_owner_id -%}

unit_tests/test_lib_charm_openstack_octavia.py

@@ -109,6 +109,15 @@ class TestOctaviaCharmConfigProperties(Helper):
         octavia.amp_boot_network_list(cls)
         self.leader_get.assert_called_with('amp-boot-network-list')
 
+    def test_spare_amphora_pool_size(self):
+        cls = mock.MagicMock()
+        self.patch('charmhelpers.core.hookenv.config', 'config')
+        self.config.return_value = None
+        self.assertEqual(octavia.spare_amphora_pool_size(cls), None)
+        self.config.return_value = 5
+        self.assertEqual(octavia.spare_amphora_pool_size(cls), 5)
+        self.config.assert_called_with('spare-pool-size')
+
 
 class TestOctaviaCharm(Helper):