core,restapi: DirectLUN disk - validate LUN visibilty

Validate LUN visibility on DirectLUN disk creation:

* core
- Added a validation method to DiskValidator that verifies
  visibility of a LUN by querying GetDeviceList.
- Execute the validation on AddDiskCommand only when a host
  is specified in parameters.
- Added relevant AppError messages.

* REST-API
- When optional 'host' element is specified within the
  'lun_storage' element, pass it to the parameters object.
- Updated rsdl yaml accordingly.

Change-Id: I339c999ebdfa69b8e2cbe4f55f8fe12e455810e4
Bug-Url: https://bugzilla.redhat.com/1123754
Buf-Url: https://bugzilla.redhat.com/1096217
Signed-off-by: Daniel Erez <derez@redhat.com>