[C++] Introduce getOptional in Settings class.
[C++] Introduce getOptional in Settings class.

Change-Id: I4ec0ba5aac2d94039066835f61c7a8b30de3b00d

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr