Configuration service uses an inconsistent hashcode in the clustered data structure

- Using Enum in the Key field of a clustered data structure can lead
to propagation issues, because the clustering services implementation
relay on a consistent hashcode to be calculated. So replaced the Java
Enum used in configuration.implementation bundle with String.

Change-Id: Id36ac46c9f38859d5325fc4f2e8674c9c3828ec4
Signed-off-by: Giovanni Meo <gmeo@cisco.com>
Merge "Configuration service uses an inconsistent hashcode in the clustered data structure"