discovery: fix print format

[gyuho]

I am getting

2017-03-10 23:00:19.229962 N | discovery: got bootstrap from DNS for etcd-server at httpmy-etcd-1.example.com:2380

[heyitsanthony]

lgtm. Worth backporting?

[fanminshi]

lgtm thanks!

[codecov-io]

Codecov Report

Merging #7482 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7482      +/-   ##
==========================================
- Coverage   70.81%   70.75%   -0.07%     
==========================================
  Files         244      244              
  Lines       21476    21476              
==========================================
- Hits        15208    15195      -13     
- Misses       5140     5153      +13     
  Partials     1128     1128

Impacted Files	Coverage Δ
discovery/srv.go	58.82% <100%> (ø)	✅
pkg/transport/timeout_conn.go	80% <0%> (-20%)	❌
pkg/adt/interval_tree.go	82.43% <0%> (-5.41%)	❌
etcdserver/cluster_util.go	73.42% <0%> (-1.4%)	❌
clientv3/watch.go	92.16% <0%> (-0.53%)	❌
etcdserver/server.go	78.68% <0%> (-0.47%)	❌
etcdserver/v3_server.go	62.5% <0%> (-0.22%)	❌
mvcc/watchable_store.go	86.43% <0%> (+0.38%)	✅
raft/node.go	90.17% <0%> (+0.44%)	✅
clientv3/client.go	78.74% <0%> (+0.78%)	✅
... and 3 more

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