containerd: specify killmode=mixed

[etungsten]

Issue #, if available: #289

Description of changes:
Changes containerd.service's KillMode from process to mixed.

KillMode=mixed is needed because when containerd.service shuts down, systemd also needs to send termination signals to its child containerd-shim processes.
Otherwise the system gets stuck on waiting for containerd-shim processes to stop when trying to reboot

Testing:
With KillMode=process (before this change):

.....
[�[0;32m  OK  �[0m] Stopped �[0;1;39mlibcontainer conta���e386221259872faea0a9e9c6a912d�[0m.
�[K[�[0;32m  OK  �[0m] Removed slice �[0;1;39mlibcontainer���_11ea_a850_02fe0b2a3c5c.slice�[0m.
[�[0;32m  OK  �[0m] Removed slice �[0;1;39mlibcontainer���iner kubepods-burstable.slice�[0m.
[�[0;32m  OK  �[0m] Removed slice �[0;1;39mlibcontainer container kubepods.slice�[0m.
[�[0;32m  OK  �[0m] Reached target �[0;1;39mShutdown�[0m.
[�[0;32m  OK  �[0m] Reached target �[0;1;39mFinal Step�[0m.
[�[0;32m  OK  �[0m] Started �[0;1;39mReboot�[0m.
[�[0;32m  OK  �[0m] Reached target �[0;1;39mReboot�[0m.
[  240.313308] systemd-shutdow: 43 output lines suppressed due to ratelimiting
[  240.322923] systemd-shutdown[1]: Syncing filesystems and block devices.
[  240.325975] systemd-shutdown[1]: Sending SIGTERM to remaining processes...
[  240.330833] systemd-journald[838]: Received SIGTERM from PID 1 (systemd-shutdow).
[  250.332130] systemd-shutdown[1]: Waiting for process: containerd-shim, containerd-shim, containerd-shim
[  330.334825] systemd-shutdown[1]: Sending SIGKILL to remaining processes...
[  330.338875] systemd-shutdown[1]: Sending SIGKILL to PID 3059 (containerd-shim).
[  330.342323] systemd-shutdown[1]: Sending SIGKILL to PID 3060 (containerd-shim).
[  330.345780] systemd-shutdown[1]: Sending SIGKILL to PID 3903 (containerd-shim).
[  330.350285] systemd-shutdown[1]: Unmounting file systems.

With KillMode=mixed:

[K[�[0;32m  OK  �[0m] Stopped �[0;1;39mlibcontainer conta���19ba2b79b2277925b84c855a2830e�[0m.
�[K[�[0;32m  OK  �[0m] Removed slice �[0;1;39mlibcontainer���_11ea_a850_02fe0b2a3c5c.slice�[0m.
[�[0;32m  OK  �[0m] Removed slice �[0;1;39mlibcontainer���iner kubepods-burstable.slice�[0m.
[�[0;32m  OK  �[0m] Removed slice �[0;1;39mlibcontainer container kubepods.slice�[0m.
[�[0;32m  OK  �[0m] Reached target �[0;1;39mShutdown�[0m.
[�[0;32m  OK  �[0m] Reached target �[0;1;39mFinal Step�[0m.
[�[0;32m  OK  �[0m] Started �[0;1;39mReboot�[0m.
[�[0;32m  OK  �[0m] Reached target �[0;1;39mReboot�[0m.
[  146.934703] systemd-shutdown[1]: Syncing filesystems and block devices.
[  146.939223] systemd-shutdown[1]: Sending SIGTERM to remaining processes...
[  146.945179] systemd-journald[864]: Received SIGTERM from PID 1 (systemd-shutdow).
[  146.980919] systemd-shutdown[1]: Sending SIGKILL to remaining processes...
[  146.986548] systemd-shutdown[1]: Unmounting file systems.

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[zmrow]

🍦