[ARM] Fix issue with deployment parameter parsing

[tjprescott]

Fixes #3729.

Ensures that --parameters <JSON String> or --parameters @<file> no longer are ignored if the contents contain an = sign. Enhances existing test to protect against regressions for this scenario.

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This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

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

Command Guidelines

• [N/A] Each command and parameter has a meaningful description.
• [N/A] Each new command has a test.

(see Authoring Command Modules)

[codecov-io]

Codecov Report

Merging #3743 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master    #3743   +/-   ##
=======================================
  Coverage   72.14%   72.14%           
=======================================
  Files         421      421           
  Lines       26009    26009           
  Branches     3943     3943           
=======================================
  Hits        18765    18765           
  Misses       6025     6025           
  Partials     1219     1219

Impacted Files	Coverage Δ
.../azure/cli/command_modules/resource/_validators.py	71.95% <50%> (ø)	⬆️

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[bencalegari]

Any idea when this will make it to a release?

[tjprescott]

Typically when the current milestone ends (which is the first week of July). We may short-circuit this one due to an unrelated fix.