Improve env init on deploying and scale-out cluster

[lucklove]

What problem does this PR solve?

At this time, we will create the user which is specified in topology.yml on deploy and scale-out stage.
However, the topology.yml only allows specifying the user name excepting the user group name. Sometimes the user may want to create the user, but using an existed group name.

BTW: fix #677

What is changed and how it works?

Add a group field in the topology file

Check List

Tests

• Integration testing

Release notes:

Support specifying user group or skipping creating a user in the deploy and scale-out stage
- to specify the group: https://github.com/pingcap/tiup/blob/master/examples/topology.example.yaml#L7
- to skip creating the user: `tiup cluster deploy/scale-out --skip-create-user xxx`

[lonng]

Please add some tests.

[codecov-commenter]

Codecov Report

Merging #678 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
+ Coverage   57.55%   57.61%   +0.05%     
==========================================
  Files         250      250              
  Lines       17932    17941       +9     
==========================================
+ Hits        10321    10336      +15     
+ Misses       6256     6250       -6     
  Partials     1355     1355              

Flag	Coverage Δ
#coverage	57.61% <ø> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...c/github.com/pingcap/tiup/pkg/cluster/api/dmapi.go	61.31% <0.00%> (-1.46%)	⬇️
...src/github.com/pingcap/tiup/pkg/cluster/manager.go	69.05% <0.00%> (ø)
...c/github.com/pingcap/tiup/pkg/cluster/spec/spec.go	90.94% <0.00%> (ø)
...m/pingcap/tiup/components/cluster/command/audit.go	84.61% <0.00%> (ø)
...ithub.com/pingcap/tiup/pkg/cluster/task/builder.go	94.09% <0.00%> (+0.04%)	⬆️
.../pingcap/tiup/components/cluster/command/deploy.go	74.46% <0.00%> (+0.55%)	⬆️
...ngcap/tiup/components/cluster/command/scale_out.go	68.08% <0.00%> (+0.69%)	⬆️
...thub.com/pingcap/tiup/pkg/cluster/task/env_init.go	57.14% <0.00%> (+1.82%)	⬆️
...github.com/pingcap/tiup/pkg/cluster/module/user.go	60.00% <0.00%> (+3.24%)	⬆️
...src/github.com/pingcap/tiup/pkg/cliutil/cliutil.go	72.05% <0.00%> (+5.88%)	⬆️
... and 1 more

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[lonng]

Rest LGTM

[lonng]

LGTM

[lonng]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@lucklove merge failed.