contexts/node: default to pod.yaml

contexts/node: default to pod.yaml

We used to default to /etc/yardstick/nodes/pod.yaml but this was changed

Recently I got a weird error

2016-11-10 01:18:03,404 - INFO - yardstick.benchmark.contexts.node - Parsing pod file: /root/isb/yardstick/
[Errno 21] Is a directory: '/root/isb/yardstick/'

It looks like if file attr is not defined we default to self.file_path =
"" which leads to a path ending with a '/' which is a directory so we
get the weird Errno 21 error.

I think we want to default to pod.yaml in YARDSTICK_ROOT_PATH

JIRA: YARDSTICK-364

Change-Id: Iab1a2b935d1e380edc208af3ce4e4758e8424bfe
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>