posix: if brick-uid or brick-gid is not specified, do not set

Current code would set owner uid/gid explicitly to 0/0 on start
even if none was specified. Fix it.

Change-Id: I72dec9e79c51bd1eb3af5334c42b7c23b01d0258
BUG: 1040275
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/6476
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Tested-by: Lukáš Bezdička <lukas.bezdicka@gooddata.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/6519
Reviewed-by: Lukáš Bezdička <lukas.bezdicka@gooddata.com>