Password rotation in secrets (#1749)

* password rotation in K8s secrets
* add db connection to syncSecrets
* add user retention
* add e2e test
* cleanup on username mismatch if rotation was switched off
* add unit test for syncSecrets + new updateSecret func

docs/reference/cluster_manifest.md

@@ -115,6 +115,22 @@ These parameters are grouped directly under  the `spec` key in the manifest.
   create the K8s secret in that namespace. The part after the first `.` is
   considered to be the user name. Optional.
 
+* **usersWithSecretRotation**
+  list of users to enable credential rotation in K8s secrets. The rotation
+  interval can only be configured globally. On each rotation a new user will
+  be added in the database replacing the `username` value in the secret of
+  the listed user. Although, rotation users inherit all rights from the
+  original role, keep in mind that ownership is not transferred. See more
+  details in the [administrator docs](https://github.com/zalando/postgres-operator/blob/master/docs/administrator.md#password-rotation-in-k8s-secrets).
+
+* **usersWithInPlaceSecretRotation**
+  list of users to enable in-place password rotation in K8s secrets. The
+  rotation interval can only be configured globally. On each rotation the
+  password value will be replaced in the secrets which the operator reflects
+  in the database, too. List only users here that rarely connect to the
+  database, like a flyway user running a migration on Pod start. See more
+  details in the [administrator docs](https://github.com/zalando/postgres-operator/blob/master/docs/administrator.md#password-replacement-without-extra-users).
+
 * **databases**
   a map of database names to database owners for the databases that should be
   created by the operator. The owner users should already exist on the cluster

docs/reference/operator_parameters.md

@@ -174,6 +174,28 @@ under the `users` key.
   Postgres username used for replication between instances. The default is
   `standby`.
 
+* **enable_password_rotation**
+  For all `LOGIN` roles that are not database owners the operator can rotate
+  credentials in the corresponding K8s secrets by replacing the username and
+  password. This means, new users will be added on each rotation inheriting
+  all priviliges from the original roles. The rotation date (in YYMMDD format)
+  is appended to the names of the new user. The timestamp of the next rotation
+  is written to the secret. The default is `false`.
+
+* **password_rotation_interval**
+  If password rotation is enabled (either from config or cluster manifest) the
+  interval can be configured with this parameter. The measure is in days which
+  means daily rotation (`1`) is the most frequent interval possible.
+  Default is `90`.
+
+* **password_rotation_user_retention**
+  To avoid an ever growing amount of new users due to password rotation the
+  operator will remove the created users again after a certain amount of days
+  has passed. The number can be configured with this parameter. However, the
+  operator will check that the retention policy is at least twice as long as
+  the rotation interval and update to this minimum in case it is not.
+  Default is `180`.
+
 ## Major version upgrades
 
 Parameters configuring automatic major version upgrades. In a