Support for per-cluster and operator global sidecars (#331)

* Define sidecars in the operator configuration.

Right now only the name and the docker image can be defined, but with
the help of the pod_environment_configmap parameter arbitrary
environment variables can be passed to the sidecars.

* Refactoring around generatePodTemplate.

Original implementation of per-cluster sidecars by @theRealWardo 

Per review by @zerg-junior and @Jan-M

docs/reference/cluster_manifest.md

@@ -213,3 +213,21 @@ properties of the persistent storage that stores postgres data.
   See [Kubernetes
   documentation](https://kubernetes.io/docs/concepts/storage/storage-classes/)
   for the details on storage classes. Optional. 
+
+### Sidecar definitions
+
+Those parameters are defined under the `sidecars` key. They consist of a list
+of dictionaries, each defining one sidecar (an extra container running
+along the main postgres container on the same pod). The following keys can be
+defined in the sidecar dictionary:
+
+* **name**
+  name of the sidecar. Required.
+
+* **image**
+  docker image of the sidecar. Required.
+
+* **env**
+  a dictionary of environment variables. Use usual Kubernetes definition
+  (https://kubernetes.io/docs/tasks/inject-data-application/environment-variable-expose-pod-information/)
+  for environment variables. Optional.

docs/reference/operator_parameters.md

@@ -15,6 +15,11 @@ words.
   your own Spilo image from the [github
   repository](https://github.com/zalando/spilo).
 
+* **sidecar_docker_images**
+  a map of sidecar names to docker images for the containers to run alongside
+  Spilo. In case of the name conflict with the definition in the cluster
+  manifest the cluster-specific one is preferred.
+
 * **workers**
   number of working routines the operator spawns to process requests to
   create/update/delete/sync clusters concurrently. The default is `4`.

docs/user.md

@@ -241,6 +241,38 @@ metadata:
 Note that timezone required for `timestamp` (offset relative to UTC, see RFC
 3339 section 5.6)
 
+
+## Sidecar Support
+
+Each cluster can specify arbitrary sidecars to run. These containers could be used for
+log aggregation, monitoring, backups or other tasks. A sidecar can be specified like this:
+
+```yaml
+apiVersion: "acid.zalan.do/v1"
+kind: postgresql
+
+metadata:
+  name: acid-minimal-cluster
+spec:
+  ...
+  sidecars:
+    - name: "container-name"
+      image: "company/image:tag"
+      env:
+        - name: "ENV_VAR_NAME"
+          value: "any-k8s-env-things"
+```
+
+In addition to any environment variables you specify, the following environment variables
+are always passed to sidecars:
+
+  - `POD_NAME` - field reference to `metadata.name`
+  - `POD_NAMESPACE` - field reference to `metadata.namespace`
+  - `POSTGRES_USER` - the superuser that can be used to connect to the database
+  - `POSTGRES_PASSWORD` - the password for the superuser
+
+The PostgreSQL volume is shared with sidecars and is mounted at `/home/postgres/pgdata`.
+
 ## Increase volume size
 
 PostgreSQL operator supports statefulset volume resize if you're using the