Fix custom config of grafana

[july2993]

fix #534 also, for Prometheus, listen on the specified host.
we call GetFreePort with the specified host.

Note that we can listen on both "0.0.0.0" and some specified ip like "127.0.0.1"

[CLAassistant]

All committers have signed the CLA.

[lucklove]

LGTM

[codecov-commenter]

Codecov Report

Merging #535 into master will increase coverage by 11.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #535       +/-   ##
===========================================
+ Coverage   39.84%   50.93%   +11.08%     
===========================================
  Files         201      220       +19     
  Lines       14847    15877     +1030     
===========================================
+ Hits         5916     8087     +2171     
+ Misses       8072     6688     -1384     
- Partials      859     1102      +243     

Flag	Coverage Δ
#coverage	50.93% <ø> (+11.08%)	⬆️

Impacted Files	Coverage Δ
...o/src/github.com/pingcap/tiup/server/store/sync.go	28.57% <0.00%> (-28.58%)	⬇️
....com/pingcap/tiup/pkg/cluster/clusterutil/retry.go	50.00% <0.00%> (-17.50%)	⬇️
go/src/github.com/pingcap/tiup/cmd/mirror.go	39.95% <0.00%> (-6.89%)	⬇️
...ingcap/tiup/components/playground/instance/tidb.go	85.36% <0.00%> (-5.34%)	⬇️
...ingcap/tiup/components/playground/instance/tikv.go	75.00% <0.00%> (-4.32%)	⬇️
...cap/tiup/components/playground/instance/tiflash.go	59.67% <0.00%> (-4.11%)	⬇️
go/src/github.com/pingcap/tiup/cmd/root.go	60.43% <0.00%> (-3.85%)	⬇️
...om/pingcap/tiup/components/cluster/command/root.go	43.03% <0.00%> (-2.24%)	⬇️
...m/pingcap/tiup/components/playground/playground.go	26.70% <0.00%> (-1.91%)	⬇️
.../pingcap/tiup/components/playground/instance/pd.go	75.43% <0.00%> (-1.23%)	⬇️
... and 113 more

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