Fix issues with the properties.yaml generation by avoiding CDumper

For some reason CDumper does not properly call the to_yaml() methods
in some situations (e.g. timestamp properties in create actions).
Not using the CDumper fixes this. Since with writing performance isn't
as crucial as with loading (because it happens less often), this should
be ok for now.
Merge branch 'jannispohlmann/fix-yaml-generation-issue'

Signed-off-by: Jannis Pohlmann <jannis.pohlmann@codethink.co.uk>