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`.