Generalize HooksMaster

- remove any dependence on Logical Units from the HooksMaster;
- add a new function parameter to the constructor, a function that is
  expected to convert the results of the hooks execution in a format
  understood by the HooksMaster;
- add a factory method that builds a HooksMaster from a LU, keeping the
  interface of the old constructor of HooksMaster;
- remove usage of Processor.hmclass from external classes, introducing
  the Processor.BuildHooksMaster method;
- update unit tests.

Reviewed-by: Michael Hanselmann <hansmi@google.com>
Signed-off-by: Andrea Spadaccini <spadaccio@google.com>