Support NodeContext type

Support NodeContext type

Initial NodeContext implementation to support BareMetal,
Controller, Compute scenarios.

Usage:

0) install yardstick
1) mkdir -p /etc/yardstick/nodes
2) cp <yardstick_repo>/etc/yardstick/nodes/pod.yaml.sample \
       /etc/yardstick/nodes/pod.yaml
3) edit /etc/yardstick/nodes/pod.yaml (make show ip, username,
       ssh key are configured correctly)
4) yardstick -d task start \
       <yardstick_repo>/samples/ping-node-context.yaml
5) cat /tmp/yardstick.out

Design etherpad link:
    https://etherpad.opnfv.org/p/yardstick_framework

JIRA: YARDSTICK-169

Change-Id: I3f6ade8243e68d88326f23ed213edb32c638ed32
Signed-off-by: QiLiang <liangqi1@huawei.com>