Distribute direct charm dependencies

Do not rely on system installed Python packages but distribute the
direct charm dependencies as part of the charms wheelhouse.

As the span of distributions we need to support with reactive
charms widens we will run into compability problems with the
current model.

For further reference see juju/charm-helpers#341 and
juju-solutions/layer-basic#135

Change-Id: Ic4e33d59039421593c8ca22f08834ba6f6561b26

src/layer.yaml

@@ -3,4 +3,3 @@ repo: https://github.com/openstack/charm-barbican-softhsm
 options:
   basic:
     use_venv: true
-    include_system_packages: true

src/wheelhouse.txt

@@ -0,0 +1,3 @@
+
+jinja2
+psutil