login: support login when there are no subscriptions

[yugangw-msft]

This is not a common thing, so users should use a flag of --allow-no-subscriptions to opt-in
Fix #2560

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• The PR has modified HISTORY.rst with an appropriate description of the change (see Modifying change log).

Command Guidelines

• Each command and parameter has a meaningful description.
  - [ ] Each new command has a test.

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[codecov-io]

Codecov Report

Merging #2929 into master will increase coverage by 0.21%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master    #2929      +/-   ##
==========================================
+ Coverage   63.11%   63.33%   +0.21%     
==========================================
  Files         466      466              
  Lines       26273    26536     +263     
  Branches     4027     4069      +42     
==========================================
+ Hits        16583    16806     +223     
- Misses       8601     8617      +16     
- Partials     1089     1113      +24

Impacted Files	Coverage Δ
...ofile/azure/cli/command_modules/profile/_params.py	66.66% <100%> (+1.96%)	⬆️
...rofile/azure/cli/command_modules/profile/custom.py	32.39% <100%> (ø)	⬆️
src/azure-cli-core/azure/cli/core/_profile.py	85.71% <91.66%> (+1.36%)	⬆️
...onitor/azure/cli/command_modules/monitor/custom.py	67.27% <0%> (ø)	⬆️
...network/azure/cli/command_modules/network/_help.py	100% <0%> (ø)	⬆️
...re-cli-vm/azure/cli/command_modules/vm/commands.py	89.53% <0%> (ø)	⬆️
...work/azure/cli/command_modules/network/commands.py	96.84% <0%> (+0.27%)	⬆️
...zure-cli-vm/azure/cli/command_modules/vm/custom.py	73.13% <0%> (+0.32%)	⬆️
...ure/cli/command_modules/network/_client_factory.py	94.02% <0%> (+0.37%)	⬆️
... and 3 more

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[tjprescott]

Why do we need an explicit flag for this? If there are subscriptions but --allow-no-subscriptions is specified, what happens? If the behavior isn't changed, then why wouldn't we just issue a warning and proceed with the --allow-no-subcriptions logic?

[yugangw-msft]

It is a bit subtle.
--allow-no-subscriptions can be used even if you have subscriptions. The story is, login will visit every tenants you are member of. Let us say you have [t1,t2], and there is a subscription s1 in t1, but not any in the t2. W/o this flag, the login will only build an account for t1\s1, which makes sense because almost all commands need to have subscriptions. But for a few commands which doesn't need subscriptions, say those ad ones, users have opened the issue saying they like to run them on t2.
I don't like to build up the tenant level accounts by default, because more likely people don't want it. That is particularly true for RBAC scenarios, that it is common people by mistake forgot to create the role assignment, causing subscription access is not built up. I am open to incoming user voices though, and can switch if needed.
Also, let me know if you have suggestions for a better arg name.