Ignore docker source cleanup err when resource not found

[darkowlzz]

Fixes the snapshot not found error seen in the CI when importing VM,
kernel images and running a VM for the first time. The issue was due to
a missing containerd snapshot key which was attempted to be cleaned up.
Since the snapshot key didn't exist, the cleanup returned snapshot <key> does not exist: not found error.
The fix checks if the error is a containerd "not found" error and
ignores it. Any other error during the cleanup is returned as before.

containerd cleanup function:

ignite/pkg/runtime/containerd/client.go

Lines 260 to 266 in fd8cd29

	cleanup = func() (err error) {
	defer util.DeferErr(&err, snapshotter.Close)
	defer util.DeferErr(&err, func() error { return snapshotter.Remove(cc.ctx, viewKey) })
	defer util.DeferErr(&err, func() error { return mount.UnmountAll(dir, 0) })
	defer util.DeferErr(&err, tarCmd.Wait)
	return
	}

In case of docker, the cleanup removes a container.

Link to a CI build with debug logs: https://travis-ci.com/github/darkowlzz/ignite/builds/160924846#L529

It's possible that we are seeing this due to the containerd dependency update from 1.3.0 to 1.3.3, or maybe this issue always existed.