Custom certs per role

- Upload the cert with `sshkit.upload!`
- Use the role name to create a directory for each role's certs
- Add an integration test for the custom certs

test/integration/docker/deployer/app_with_custom_certificate/Dockerfile

@@ -0,0 +1,10 @@
+FROM registry:4443/nginx:1-alpine-slim
+
+COPY default.conf /etc/nginx/conf.d/default.conf
+
+ARG COMMIT_SHA
+
+RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
+RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
+RUN mkdir -p /usr/share/nginx/html/versions && echo "hidden" > /usr/share/nginx/html/versions/.hidden
+RUN echo "Up!" > /usr/share/nginx/html/up