embed: use '*url.URL.Hostname(),Port()' for Go 1.8

[gyuho]

Wherever net.SplitHostPort is used and we don't care about the parsing error, we can just use the new methods Hostname and Port that strip out the port or hostname.

https://golang.org/doc/go1.8#net_url

[codecov-io]

Codecov Report

Merging #7766 into master will decrease coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #7766     +/-   ##
=========================================
- Coverage   76.05%   75.85%   -0.2%     
=========================================
  Files         331      331             
  Lines       26053    26065     +12     
=========================================
- Hits        19814    19772     -42     
- Misses       4821     4869     +48     
- Partials     1418     1424      +6

Impacted Files	Coverage Δ
etcdserver/raft.go	88.39% <100%> (+0.24%)	⬆️
embed/config.go	70.48% <100%> (ø)	⬆️
etcdserver/server.go	80.25% <100%> (+0.36%)	⬆️
proxy/grpcproxy/register.go	80.55% <0%> (-13.89%)	⬇️
pkg/adt/interval_tree.go	77.74% <0%> (-11.9%)	⬇️
pkg/fileutil/purge.go	73.68% <0%> (-7.9%)	⬇️
auth/simple_token.go	86.79% <0%> (-6.61%)	⬇️
proxy/grpcproxy/watch.go	89.58% <0%> (-4.87%)	⬇️
pkg/testutil/recorder.go	66.66% <0%> (-3.71%)	⬇️
etcdserver/api/v3rpc/lease.go	96.42% <0%> (-3.58%)	⬇️
... and 13 more

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