NodePort information returned in context should be a dictionary

NodePort information returned in context should be a dictionary

In Kubernetes context, NodePort information returned in context should be
a dictionary. In [1], the node information is dumped in a yaml file. The
NodePort object doesn't have a serialize implementation; instead of
returning the object, an equivalent dictionary should be returned.

[1]https://github.com/opnfv/yardstick/blob/f6fa0d7422f0669d049dbf07a51eb39b1dc25830/yardstick/benchmark/scenarios/networking/vnf_generic.py#L293

JIRA: YARDSTICK-1323

Change-Id: If4844c9b074920a7a7d8c1a424b681f9f8a4b0c5
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>