rebase: update golang version to v.1.15
rebase: update golang version to v.1.15

as the  golang v1.15 is released,updated cephcsi
golang version to 1.15

kubernetes 1.19 is going to be built with
golang 1.15 see https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.19.md#api-change
Signed-off-by: Madhu Rajanna <60fbf37a92c2a0908e31296d18dadbec5a4dae17@gmail.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi