Use custom YAML subclass to be compatible with ruamel_yaml>=0.17

Signed-off-by: Martin Weinelt <hexa@darmstadt.ccc.de>

whipper/common/yaml.py

@@ -0,0 +1,18 @@
+from ruamel.yaml import YAML as ruamel_YAML
+from ruamel.yaml.compat import StringIO
+
+# https://yaml.readthedocs.io/en/latest/example.html#output-of-dump-as-a-string
+class YAML(ruamel_YAML):
+    def __init__(self, *args, **kwargs):
+        super().__init__()
+        self.width = 4000
+        self.default_flow_style = False
+
+    def dump(self, data, stream=None, **kw):
+        inefficient = False
+        if stream is None:
+            inefficient = True
+            stream = StringIO()
+        ruamel_YAML.dump(self, data, stream, **kw)
+        if inefficient:
+            return stream.getvalue()