Check that architecture config doesn't include "all"

Add 'all' as an architecture will not give the correct result. To avoid
confusion, explicitly check for this and error out if it is added.

britney2/inputs/suiteloader.py

@@ -19,6 +19,8 @@ class SuiteContentLoader(object):
     def __init__(self, base_config):
         self._base_config = base_config
         self._architectures = SuiteContentLoader.config_str_as_list(base_config.architectures)
+        assert 'all' not in self._architectures, "all not allowed in architectures"
+
         self._nobreakall_arches = SuiteContentLoader.config_str_as_list(base_config.nobreakall_arches, [])
         self._outofsync_arches = SuiteContentLoader.config_str_as_list(base_config.outofsync_arches, [])
         self._break_arches = SuiteContentLoader.config_str_as_list(base_config.break_arches, [])