etcdmain: SdNotify when gateway, grpc-proxy are ready

[gyuho]

Just tried this on Container Linux with systemd, confirmed that it works well without sd-notify errors.

Mar 09 17:52:19 gyuho-coreos.c.etcd-development.internal systemd[1]: Starting etcd...
Mar 09 17:52:19 gyuho-coreos.c.etcd-development.internal etcd[25737]: ready to proxy client requests to [127.0.0.1:2379]
Mar 09 17:52:19 gyuho-coreos.c.etcd-development.internal systemd[1]: Started etcd.

Mar 09 17:54:06 gyuho-coreos.c.etcd-development.internal systemd[1]: Starting etcd...
Mar 09 17:54:06 gyuho-coreos.c.etcd-development.internal etcd[25832]: listening for grpc-proxy client requests on 127.0.0.1:23791
Mar 09 17:54:06 gyuho-coreos.c.etcd-development.internal systemd[1]: Started etcd.

Fix #7465

[heyitsanthony]

this is duplicated three times, consolidate into:

func notifySystemd() {
    if !systemdutil.IsRunningSystemd() {
       return
    }
    ...
}

in main.go?

[gyuho]

@heyitsanthony PTAL? Thanks!

[heyitsanthony]

lgtm. Thanks!

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@ad1d48b). Click here to learn what that means.
The diff coverage is 45.45%.

@@           Coverage Diff            @@
##             master   #7467   +/-   ##
========================================
  Coverage          ?   70.8%           
========================================
  Files             ?     244           
  Lines             ?   21437           
  Branches          ?       0           
========================================
  Hits              ?   15178           
  Misses            ?    5132           
  Partials          ?    1127

Impacted Files	Coverage Δ
etcdmain/gateway.go	35.08% <0%> (ø)
etcdmain/grpc_proxy.go	19.81% <0%> (ø)
etcdmain/etcd.go	45.33% <100%> (ø)
etcdmain/main.go	50% <50%> (ø)

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