Reimplement Setting#POST endpoint

[adamczykba]

Describe the bug
POST endpoint in SettingsResource aside from creating new records also overwrites existing ones. There should be some check for duplicates.

To Reproduce
Steps to reproduce the behavior:

1. Create new Setting through REST request
2. Send the same request second time (optionally try to change some value to see the change)
3. Go to Settings table in DB and see that record has been changed

Expected behavior
POST endpoint should only create new records. It should not be able to update them.

Observed behavior
SettingsResource#POST request overwrites existing settings records

Screenshots
N/A

Additional context
N/A

[Crystalzord]

I already commented on this here: #164
The implementation of saveConfigurationSetting method from OperatorServiceBean class in the oscm repository and the ConfigurationKey class from oscm-interfaces need refactoring in order to make the POST endpoint work correctly.

I edited the task name.