Fix Bug 2028 Multi-site VNF creation fails

This fix will provide to use  the vim_network_name from instantiation parameters if it exists,
if not, network which is given in NSD will be used for the NS which is instantiated on multi-site VIMs.

Change-Id: Ie39e3444428474c8db01d59ef1a1995644ea6a81
Signed-off-by: aticig <gulsum.atici@canonical.com>