Allow setting the cluster readOnly

[calind]

We should add a spec.readOnly which would set the entire cluster read-only (even the master node).

The controller should make sure that once the readOnly condition in status is set to true, there is no way the master becomes writable. It is ok to have a spec.readOnly=false cluster with all it's pod read-only, but it's a bug to have a spec.readOnly=true and status.conditions[readOnly]=true with writable pods.

For this we need to do the following:

1. all cluster pods start read-only
2. the controller is responsible of making the master pod writable
3. make sure that orchestrator is started with ApplyMySQLPromotionAfterMasterFailover set to false

[MerceaOtniel]

Addressed #100