Use tox in Makefile targets

Modify the Makefile to point at the appropriate tox targets
so that tox and Make output can be equivalent. This involves
mapping the lint target to the pep8 target and the test target
to the py27 target.

Update tox.ini to not use site-packages as the PyYAML installed
version conflicts with pip requirements.

Change-Id: I32c2829af2a44e25c429bb603098a1249d415356

Makefile

@@ -2,14 +2,11 @@
 PYTHON := /usr/bin/env python
 
 lint:
-	@flake8 --exclude hooks/charmhelpers,tests/charmhelpers \
-        actions hooks unit_tests tests
-	@charm proof
+	@tox -e pep8
 
 test:
-	@# Bundletester expects unit tests here.
 	@echo Starting unit tests...
-	@$(PYTHON) /usr/bin/nosetests -v --nologcapture --with-coverage unit_tests
+	@tox -e py27
 
 functional_test:
 	@echo Starting Amulet tests...

tox.ini

@@ -8,7 +8,6 @@ setenv = VIRTUAL_ENV={envdir}
 install_command =
   pip install --allow-unverified python-apt {opts} {packages}
 commands = ostestr {posargs}
-sitepackages = True
 
 [testenv:py27]
 basepython = python2.7