Use the RSA module explicitly

[narrieta]

Reverting #1552

Reopened #1550

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[narrieta]

#1552 added unit tests for this path

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Codecov Report

❗ No coverage uploaded for pull request base (develop@83f0f95). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             develop    #1629   +/-   ##
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  Coverage           ?   66.51%           
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  Files              ?       78           
  Lines              ?    11182           
  Branches           ?     1570           
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  Hits               ?     7438           
  Misses             ?     3414           
  Partials           ?      330

Impacted Files	Coverage Δ
azurelinuxagent/common/utils/cryptutil.py	50% <100%> (ø)

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[pgombar]

LGTM

[larohra]

LGTM

[johncrim]

I object to this change - would be best to use pkey when available, and rsa only for older versions of openssl.

[narrieta]

@johncrim - yes, we need to add code to collect some data to help us determine what is the best way to fallback to rsa when pkey is not available. We'll do that on the next release