gluster: Fixed volume rebalance status started time

Corrected to set the correct value of the started at field value for the
task started for rebalance of gluster volumes.

Change-Id: Ie7b22d988e70c6cc58e7cb0b74263d2e09f772ff
Bug-Url: https://bugzilla.redhat.com/1011878
Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>