Change openstack clients install directive

This change switches to using the python_packages *1 layer.yaml
directive for installing the openstack clients for use by the
charm. This is primarily to work around *2

*1 https://charmsreactive.readthedocs.io/en/latest/layer-basic.html#layer-configuration
*2 https://github.com/juju-solutions/layer-basic/issues/149

Change-Id: I618235384164c03776bd25bf669543fbd99794ad

src/layer.yaml

@@ -3,3 +3,4 @@ options:
   basic:
     use_venv: True
     include_system_packages: False
+    python_packages: ['keystoneauth1', 'python-glanceclient', 'python-novaclient', 'python-neutronclient']

src/wheelhouse.txt

@@ -1,5 +0,0 @@
-keystoneauth1
-pbr
-python-glanceclient
-python-novaclient
-python-neutronclient