nmp.sh: move to directory with downloaded models
nmp.sh: move to directory with downloaded models

Because of dependencies between some downloaded models, we need to
change working path of python. Thanks to that, pyang(1) is able to find
dependencies.

CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI