Bug fix: vping_userdata test status was not properly reported

Bug fix: vping_userdata test status was not properly reported

vping_base is calling vping_userdata
vping_user data could return only EX_TESTCASE_FAILED or EX_OK
by default vping_base always returns EX_OK (based on self.result=100..)
it returned EX_RUN_ERROR only if upper class report EX_RUN_ERROR

that is why vping_userdata was always PASS even if the trace "Ping OK"
cannot be retrieved from the Nova console

it seems that when running the vping_userdata, we systematically got
 Failed to read iid from metadata

Change-Id: If72441dd93bffada5c7a5399c87db68d7ebeebed
Signed-off-by: Morgan Richomme <morgan.richomme@orange.com>
(cherry picked from commit e36a14f6db0bc5ac93469bb553810de506f29bef)