Fix false message instance is down

Fix false message instance is down

JIRA: SDNVPN-238
JIRA: SDNVPN-232

This patch fixes the false message instance is down. The message
occurs because a wrong method was used to check if instance is
alive. This instance does not proceed to login so the previous
method cound not find "login" regex inside its console output.

Change-Id: I35733f634595a9d2eeb9148c2476444afab710a6
Signed-off-by: Stamatis Katsaounis <mokats@intracom-telecom.com>
(cherry picked from commit eea8b6c296f7c9540de7f46bc9daaee9e1e14ae5)