Avoid overwriting critical users. (#172)

* Avoid overwriting critical users.

Disallow defining new users either in the cluster manifest, teams
API or infrastructure roles with the names mentioned in the new
protected_role_names parameter (list of comma-separated names)

Additionally, forbid defining a user with the name matching either
super_username or replication_username, so that we don't overwrite
system roles required for correct working of the operator itself.

Also, clear PostgreSQL roles on each sync first in order to avoid using
the old definitions that are no longer present in the current manifest,
infrastructure roles secret or the teams API.

README.md

@@ -208,6 +208,8 @@ The following steps will get you the docker image built and deployed.
 of configuration parameters applied to the roles fetched from the API.
 For instance, `team_api_role_configuration: log_statement:all,search_path:'public,"$user"'`.
 By default is set to *"log_statement:all"*. See [PostgreSQL documentation on ALTER ROLE .. SET](https://www.postgresql.org/docs/current/static/sql-alterrole.html) for to learn about the available options.
+* protected_role_names - a list of role names that should be forbidden as the manifest, infrastructure and teams API roles.
+The default value is `admin`. Operator will also disallow superuser and replication roles to be redefined.
 
 
 ### Debugging the operator itself