meta: add .file_write method into meta_ops

Add .file_write method in meta_ops and make write support into
virtual files (like loglevel, measure_latency) much simpler.

Change-Id: I812f782f645605c56f81b158bb0f289b3c8f909a
BUG: 1089216
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/7785
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>