ping: don't split if target_vm is a dict

ping: don't split if target_vm is a dict

If we run sample/ping-hot.yaml, it will encounter an AttributeError, log
see below:
Traceback (most recent call last):
File
"/usr/local/lib/python2.7/dist-packages/yardstick/benchmark/runners/duration.py",
line 69, in _worker_process
method(data)
File
"/usr/local/lib/python2.7/dist-packages/yardstick/benchmark/scenarios/networking/ping.py",
line 94, in run
target_vm_name = target_vm.split('.')[0]
AttributeError: 'dict' object has no attribute 'split'
Because here host and target will be a dict

JIRA: YARDSTICK-561

Change-Id: I4b7628bf20050d6d516a80efe3785f750d27c05e
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>