sdk: Replace decorators before marshal

The code generated by the generateDS.py tool expects that all the
objects implement the "export" method that generates the XML text. The
entity and collection decorators don't implement this methods, but the
user may use them in places where simple parameter objects are expected.
To avoid issues with this we had an implementation of the "__setattr__"
method in the base class of parameter objects that replaces references
to decorators with the corresponding parameter objects. But this doesn't
work when the references are set without calling the __setattr__ method,
for example when adding an element to a list. To avoid this issue this
patch removes the __setattr__ method and instead of that the replacement
is executed recusively right before doing the conversion to XML.

Change-Id: I7843dea85c9f09de5fb3439f57dba71ce6a28898
Bug-Url: https://bugzilla.redhat.com/1024696
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>