Make unable to ping log messages appropriate level

The ping from an ssh connection checks in _check_remote_connectivity()
aren't necessarily expected to pass on the first attempt. However,
every time the ping was unsuccessful the method would log an
exception which is too severe a log for an expected error. This
commit uses a more appropriate log level for this case.

Change-Id: I8068fa85472259676acebd99b9d191065661e101
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index 07d8828..b207161 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -943,7 +943,8 @@
             try:
                 source.ping_host(dest)
             except exceptions.SSHExecCommandFailed:
-                LOG.exception('Failed to ping host via ssh connection')
+                LOG.warn('Failed to ping IP: %s via a ssh connection from: %s.'
+                         % (dest, source.ssh_client.host))
                 return not should_succeed
             return should_succeed