Add OpenFaaS addon (#1839)

**What**
- Create addon that installs openfaas using helm3. The user can
  toggle if the installation should enable basic auth and if
  it should install the operator mode instead of the standard
  controller.  All other settings can be controlled via an
  external values files that can be passed as a flag.

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

Co-authored-by: Konstantinos Tsakalozos <kos.tsakalozos@canonical.com>

tests/test-addons.py

@@ -28,6 +28,7 @@ from validators import (
     validate_traefik,
     validate_coredns_config,
     validate_portainer,
+    validate_openfaas,
 )
 from utils import (
     microk8s_enable,
@@ -357,6 +358,17 @@ class TestAddons(object):
         print("Disabling Portainer")
         microk8s_disable("portainer")
 
+    def test_openfaas(self):
+        """
+        Sets up and validates OpenFaaS.
+        """
+        print("Enabling openfaas")
+        microk8s_enable("openfaas")
+        print("Validating openfaas")
+        validate_openfaas()
+        print("Disabling openfaas")
+        microk8s_disable("openfaas")
+
     def test_traefik(self):
         """
         Sets up and validates traefik.