Add tox.ini for building and flake8 tests

Add a tox.ini for creating virtualenvs and then executing builds,
flake8, unit tests and charm proof in those venvs. Example:

tox -e pep8
tox -e build
tox -e charm-proof
tox -e unit-tests

Partial-Bug: #1745966

.gitignore

@@ -1 +1,5 @@
 *~
+build/*
+.tox/*
+.stestr/*
+__pycache__

.stestr.conf

@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=./unit_tests
+top_dir=./

requirements.txt

@@ -0,0 +1,2 @@
+# Requirements to build the charm
+charm-tools

test-requirements.txt

@@ -0,0 +1,4 @@
+flake8>=2.2.4,<=2.4.1
+os-testr>=0.4.1
+mock>=1.2
+coverage>=3.6

tox.ini

@@ -0,0 +1,44 @@
+[tox]
+skipsdist = True
+envlist = pep8,unit-tests
+skip_missing_interpreters = True
+
+[testenv]
+setenv = VIRTUAL_ENV={envdir}
+         PYTHONHASHSEED=0
+         TERM=linux
+         LAYER_PATH={toxinidir}/layers
+         INTERFACE_PATH={toxinidir}/interfaces
+         JUJU_REPOSITORY={toxinidir}/build
+passenv = http_proxy https_proxy USER
+install_command =
+  pip install {opts} {packages}
+deps =
+    -r{toxinidir}/requirements.txt
+
+[testenv:build]
+basepython = python2.7
+commands =
+    charm-build --log-level DEBUG -o {toxinidir}/build . {posargs}
+
+[testenv:pep8]
+basepython = python3
+deps = -r{toxinidir}/test-requirements.txt
+commands = flake8  --exclude build {posargs} .
+
+[testenv:unit-tests]
+basepython = python3
+deps = -r{toxinidir}/test-requirements.txt
+commands = ostestr {posargs}
+
+[testenv:venv]
+commands = {posargs}
+
+[flake8]
+# E402 ignore necessary for path append before sys module import in actions
+ignore = E402
+
+[testenv:charm-proof]
+basepython = python2.7
+toxworkdir = {toxinidir}/build/builds/vault
+commands = charm-proof