Print default plugin config.

[Random-Liu]

Fixes #2027.

@containerd/containerd-maintainers Does this make sense to you?

The output:

$ containerd config default
root = "/var/lib/containerd"
state = "/run/containerd"
oom_score = 0

[grpc]
  address = "/run/containerd/containerd.sock"
  uid = 0
  gid = 0

[debug]
  address = "/run/containerd/debug.sock"
  uid = 0
  gid = 0
  level = "info"

[metrics]
  address = ""
  grpc_histogram = false

[cgroup]
  path = ""

[plugins]
  [plugins.cgroups]
    no_prometheus = false
  [plugins.diff]
    default = ["walking"]
  [plugins.linux]
    shim = "containerd-shim"
    runtime = "runc"
    runtime_root = ""
    no_shim = false
    shim_debug = false
  [plugins.scheduler]
    pause_threshold = 0.02
    deletion_threshold = 0
    mutation_threshold = 100
    schedule_delay = 0
    startup_delay = 100000000

Signed-off-by: Lantao Liu lantaol@google.com

[codecov-io]

Codecov Report

Merging #2030 into master will decrease coverage by 4.58%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2030      +/-   ##
==========================================
- Coverage   50.17%   45.58%   -4.59%     
==========================================
  Files          84       94      +10     
  Lines        7342     9296    +1954     
==========================================
+ Hits         3684     4238     +554     
- Misses       2952     4348    +1396     
- Partials      706      710       +4

Flag	Coverage Δ
#linux	50.21% <0%> (+0.04%)	⬆️
#windows	40.54% <0%> (?)

Impacted Files	Coverage Δ
server/config.go	0% <ø> (ø)	⬆️
server/server.go	2.51% <0%> (-0.52%)	⬇️
fs/path.go	60.69% <0%> (-12.56%)	⬇️
snapshots/naive/naive.go	43.89% <0%> (-10%)	⬇️
metadata/snapshot.go	45.95% <0%> (-8.96%)	⬇️
archive/compression/compression.go	43.93% <0%> (-8.9%)	⬇️
remotes/docker/fetcher.go	41.42% <0%> (-7.6%)	⬇️
content/local/writer.go	52.63% <0%> (-7.37%)	⬇️
fs/copy.go	34.83% <0%> (-7.2%)	⬇️
archive/tar.go	43.51% <0%> (-6.94%)	⬇️
... and 59 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 823d339...3d6fe5a. Read the comment docs.

[crosbymichael]

Why use a buffer here and not just w?

[Random-Liu]

Copied from original code. Will remove it then.

[Random-Liu]

Done.

[Random-Liu]

@crosbymichael Addressed comments.

[crosbymichael]

LGTM

[Random-Liu]

Fixed a vet error.

[mlaventure]

LGTM