etcdserver: Missing the cfg.Logger causes panic

[needkane]

Please read https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#contribution-flow.

In "etcdserver/server.go"，function “applyEntryNormal” calls “applyV2Request“ ，and “applyV2Request” calls “Put” when etcd application starts.

There is log output in this function ("Put"), or log will be added later, but "a.lg" is nil when function is executed causes panic.

Test :

You can add ' fmt.Println(a.lg, "-----------applierV2store Put 67--------") ' to the
'func (a *applierV2store) Put(r *RequestV2) Response'

[needkane]

/cc @jingyih @gyuho

[gyuho]

lgtm thx