Remove redundant certificates interface and layer

We now consume the default handlers for certificates from the
``openstack`` layer with backing code in ``charms.openstack``.

Also fixup charm tools development environment variables for local
layer and interface repositories.

src/layer.yaml

@@ -1,8 +1,6 @@
 includes:
   - layer:leadership
   - layer:openstack
-  - layer:tls-client
-  - interface:tls-certificates
 options:
   basic:
     use_venv: True

src/metadata.yaml

@@ -27,9 +27,6 @@ series:
   - bionic
   - disco
 subordinate: false
-requires:
-  certificates:
-    interface: tls-certificates
 provides:
   ovsdb:
     interface: ovsdb

tox.ini

@@ -9,7 +9,8 @@ envlist = pep8,py3
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
          TERM=linux
-         LAYER_PATH={toxinidir}/layers
+         CHARM_LAYERS_DIR={toxinidir}/layers
+         CHARM_INTERFACES_DIR={toxinidir}/interfaces
          JUJU_REPOSITORY={toxinidir}/build
 passenv = http_proxy https_proxy INTERFACE_PATH
 install_command =