Add wait/no-wait support to "az disk" commands.

[tjprescott]

Closes #2215. Partial addresses #1528.

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General Guidelines

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

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• Each command and parameter has a meaningful description.
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[codecov-io]

Codecov Report

Merging #3390 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3390      +/-   ##
=========================================
- Coverage    70.7%   70.7%   -0.01%     
=========================================
  Files         393     393              
  Lines       25472   25474       +2     
  Branches     3884    3884              
=========================================
+ Hits        18010   18011       +1     
- Misses       6319    6320       +1     
  Partials     1143    1143

Impacted Files	Coverage Δ
...azure-cli-vm/azure/cli/command_modules/vm/_help.py	100% <100%> (ø)	⬆️
...re-cli-vm/azure/cli/command_modules/vm/commands.py	89.59% <100%> (+0.06%)	⬆️
...zure-cli-vm/azure/cli/command_modules/vm/custom.py	73.36% <100%> (ø)	⬆️
...li-rdbms/azure/cli/command_modules/rdbms/custom.py	63.93% <0%> (-1.64%)	⬇️

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[yugangw-msft]

LGTM. Let @johanste provides the query to verify we don't have many users using the disk delete. Once confirmed, please go ahead and merge

[tjprescott]

@yugangw-msft after looking at the usage statistics, are you comfortable with the breaking change or should be hold off on adding the prompt?