Check options to verify that we don't have duplicated keys in the descriptors

[antonroman]

Right now it is possible to define to deployment_flavours keys in the NSD. This doesn't trigger any error in the schema validation and the parsing library only gives the last value.
This could lead to problems if the developer doesn't realize that some key is duplicated and it's only looking at the values of the first one. In order to prevent this from happening we are going to explore options to check the duplicates by reading the descriptor file directly as text or (preferably) using other existing library.