Test Postgres cluster resource

[ribaraka]

We need to test all Postgres spec fields, and settings, check their workability, create/update/delete operations, add/remove data centers, etc.

Create separate issues for all bugs found during the test process. If not, add your comment on what was tested and the results in this issue.

• make dataCentres.interDataCentreReplication and dataCentres.intraDataCentreReplication immutable during resource update #662
• handle external changes of clusterConfiguration #729

[worryg0d]

I found the following issues:

• The operator allows updating dataCentres.interDataCentreReplication.isPrimaryDataCentre field, but when an update request is sent to the Instaclustr API it returns a response with status code 400 and says that the update is not supported.
• With dataCentres.intraDataCentreReplication.replicationMode the same story. Also, the operator allows to set any value to the field but only ASYNCHRONOUS and SYNCHRONOUS are available.
• Customers may update clusterConfigurations via Instaclustr Console, but the operator doesn't handle changes for clusterConfigurations field. It should drop to external changes state