Use tox to test the python3-flask template

**What**
- Add a sample tox.ini file to the python3-flask. This configuration
  will run pytest and flake8 for the function handler. The default
  flake8 is very resticted, only erroring for undefined names and
  syntax errors. This allows us to lint and test the function during
  `faas-cli build`, which should simplify CI flows for users. Using
  `tox` allows th euser to completely customize the test flow without
  requiring any changes to the template.
- Add same test for the echo handler
- Include documentationthat describes how to disable

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

template/python3-flask/Dockerfile

@@ -34,13 +34,14 @@ WORKDIR /home/app/function/
 COPY function/requirements.txt	.
 RUN pip install --user -r requirements.txt
 
-WORKDIR /home/app/
-
 #install function code
 USER root
 
-COPY function   function
-RUN chown -R app:app ./
+COPY function/   .
+RUN chown -R app:app ../
+RUN  tox -l == 0 && tox
+
+WORKDIR /home/app/
 
 #configure WSGI server and healthcheck
 USER app

template/python3-flask/function/handler_test.py

@@ -0,0 +1,8 @@
+from .handler import handle
+
+# Test your handler here
+# To disable testing, you need to update the
+# tox.ini file, in addition to deleting this file.
+
+def test_handle():
+    assert handle("input") == "input"

template/python3-flask/function/tox.ini

@@ -0,0 +1,41 @@
+# If you would like to disable
+# automated testing during faas-cli build,
+
+# Replace the content of this file with
+#   [tox]
+#   skipsdist = true
+
+# You can also edit, remove, or add additional test steps
+# by editing, removing, or adding new testenv sections
+
+
+# find out more about tox: https://tox.readthedocs.io/en/latest/
+[tox]
+envlist = lint,test
+skipsdist = true
+
+[testenv:test]
+deps =
+  flask
+  pytest
+  -rrequirements.txt
+commands =
+  # run unit tests with pytest
+  # https://docs.pytest.org/en/stable/
+  # configure by adding a pytest.ini to your handler
+  pytest
+
+[testenv:lint]
+deps =
+  flake8
+commands =
+  flake8 .
+
+[flake8]
+count = true
+max-line-length = 127
+max-complexity = 10
+statistics = true
+# stop the build if there are Python syntax errors or undefined names
+select = E9,F63,F7,F82
+show-source = true

template/python3-flask/requirements.txt

@@ -1,3 +1,3 @@
 flask
 waitress
-
+tox==3.*