Give AmpliconSampleSheet a SheetType.

Give AmpiconSampleSheet a SheetType and SheetVersion, since it's
expected that children of KLSampleSheet have them defined. Dummy
amplicon sample-sheets are created by SPP to pass to bcl-convert, based
on the lab-supplied pre-prep file. Giving it the type 'dummy_amp' avoids
having it erroneously say 'standard_metag'.

metapool/sample_sheet.py

@@ -15,6 +15,7 @@
 _METAGENOMIC = 'Metagenomic'
 _METATRANSCRIPTOMIC = 'Metatranscriptomic'
 _STANDARD_SHEET_TYPE = 'standard_metag'
+_DUMMY_SHEET_TYPE = 'dummy_amp'
 _ABSQUANT_SHEET_TYPE = 'abs_quant_metag'
 SHEET_TYPES = (_STANDARD_SHEET_TYPE, _ABSQUANT_SHEET_TYPE)
 
@@ -711,7 +712,7 @@ def _validate_sample_sheet_metadata(self, metadata):
 class AmpliconSampleSheet(KLSampleSheet):
     _HEADER = {
         'IEMFileVersion': '4',
-        'SheetType': _STANDARD_SHEET_TYPE,
+        'SheetType': _DUMMY_SHEET_TYPE,
         'SheetVersion': '0',
         'Investigator Name': 'Knight',
         'Experiment Name': 'RKL_experiment',