catapult: make GenericSetDiagnostic values interface{}
GenericSetDiagnostic is allowed to hold any value type. We need to
support integer values because the Catapult dashboard expects the
chromiumCommitPositions diagnostic to contain an integer value.
IN-199
Change-Id: I29f4c7afa0ff33023893b02f3f0b0095b68fa1c9