core: fluent interface/method chaining for addCustomValue

basically this means we could:

  AuditLogDirector.log(
      new AuditLogableBase()
          .addCustomValue(...)
          .addCustomValue(...)
          .addCustomValue(...),
          LogType...)

Change-Id: Ibba14b4dd5ad520ea452ea940e9106c28cc5e4d4
Signed-off-by: Roy Golan <rgolan@redhat.com>