External key value config store

[svaroqui]

Enable saving and restoring per cluster config variables from various datastore.

At minimum to implement a first datastore using the replication-manager-pro arbitrator instance that can save data in embedded sqllite or external MariaBD RDBMS

In that first step to offer to replication-manager-pro arbitrator instead of different binary , should save some precious compile time and different packages installation ( osc release should complain for support license issue)

The next datastore nice to have is an etcd cluster made out of 3 different replication-manager-pro monitor instances

We should unify all this with already existing code (monitoring-save-config)

• to save and restore per cluster config variables changes inside a toml file store inside the datadir of the cluster
• to restore per cluster config variables from env variables

Keep in mind that the default section and command line parameters should be seen as a way to enforce some variables settings for all clusters and should be proceed and merge at the last step

Today it does not exist any code facility to tell if a variable was change from it's default and it would be nice to have in same PR