Depend on 'juju' when no extra is passed.

By default depend on 'juju' (no pinning), the pinning only comes into
place when an explicit extra is passed.

This makes the extras effectively nothing more than a pinning alias,
basically `pip install juju<3.0 zaza` becomes equivalent to `pip install
zaza[juju-29]`.

setup.py

@@ -48,6 +48,7 @@
 
 extras_require={
     'testing': tests_require,
+    '': ['juju'],
     'juju-29': ['juju<3.0'],
     'juju-31': ['juju>=3.1.0,<3.2.0'],
     'juju-32': ['juju>=3.2.0,<3.3.0'],

tox.ini

@@ -17,7 +17,7 @@ passenv =
   CS_*
   OS_*
   TEST_*
-extras = {env:EXTRA:juju-29}
+extras = {env:EXTRA}
 deps =
   -r{toxinidir}/test-requirements.txt
 commands = pytest --cov=./zaza/ {posargs} {toxinidir}/unit_tests