Revert "mdadm/systemd: remove KillMode=none from service file"

Revert "mdadm/systemd: remove KillMode=none from service file"

This reverts commit 52c67fcdd6dadc4138ecad73e65599551804d445.

The functionality is marked as deprecated but we don't have alternative
solution yet. Shutdown hangs if OS is installed on external array:

task:umount state:D stack: 0 pid: 6285 ppid: flags:0x00004084
Call Trace:
__schedule+0x2d1/0x830
? finish_wait+0x80/0x80
schedule+0x35/0xa0
md_write_start+0x14b/0x220
? finish_wait+0x80/0x80
raid1_make_request+0x3c/0x90 [raid1]
md_handle_request+0x128/0x1b0
md_make_request+0x5b/0xb0
generic_make_request_no_check+0x202/0x330
submit_bio+0x3c/0x160

Use it until new solution is implemented.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>