glusterd: suppress spurious error message during startup

From glusterd log:
----------
E [glusterd-store.c:1981:glusterd_store_retrieve_volume] 0-: Unknown key:
brick-0
----------

The message is emitted from glusterd_store_retrieve_volume() when it reads the
volinfo file because it doesn't do anything with the key-value pair. Suppress the
error. The key is needed by glusterd_store_retrieve_bricks() which anyway
re-reads it.

Also change the log level to WARNING since we do not error out if an unknown key
is got while parsing the volinfo file.

Change-Id: Icd7962d9e16e0f90e6a37ee053dcafe97d2cab94
BUG: 1079279
Reviewed-on: http://review.gluster.org/7314
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>