If log level is already set via api or command line, initialization of
io-stats xlator overwrites the log level to GF_LOG_INFO. This patch
prevents re-initialization of log level if already set.

Backport of:
> Change-Id: I1f74d94ef8068b95ec696638c0a8b17d8d71aabe
> BUG: 1368882
> Signed-off-by: Vijay Bellur <vbellur@redhat.com>
> Reported-by: Colin Lord <clord@redhat.com>
> Reviewed-on: http://review.gluster.org/15112
> NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
> Smoke: Gluster Build System <jenkins@build.gluster.org>
> CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
> Reviewed-by: Niels de Vos <ndevos@redhat.com>
> Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>

Change-Id: I1f74d94ef8068b95ec696638c0a8b17d8d71aabe
BUG: 1378384
Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Reviewed-on: http://review.gluster.org/15544
Smoke: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>