Flush stdout as it can be redirected to log

Flush stdout as it can be redirected to log

It avoids mixed logs in Jenkins' console by flushing every message
printed by the forked programs as logger.info() does.
It must be complete by refactoring logger management in util functions
[1] and by preventing output buffering in forked programs.

[1] https://jira.opnfv.org/browse/FUNCTEST-428

JIRA: FUNCTEST-441

Change-Id: Id8da814d79ea6b1e6b91b49e23a85c3802e11515
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit c7edada4ccf84851b76b9ef738c270a7a6819a06)