Refactor start_extension_command to handle subprocess completion or timeout

[pgombar]

Description

Before, it was the responsibility of ExtHandlerInstance's launch_command to wait for the subprocess to terminate or timeout, parse its output and raise exceptions if needed.

Since the actual invocation of the subprocess is done in the CGroup API layer, if systemd-run times out, we wouldn't know until the ExtHandlerInstance layer, at which point we wouldn't retry invoking the subprocess using Popen and we would erroneously raise an ExtensionError which would be counted against the extension's SLA.

This PR refactors the code so that these responsibilities are done in the start_extension_command in the CGroup API layer instead of inlaunch_command. Helper functions for waiting for process completion are added to process_utils.py and corresponding unit tests are added or updated.

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This change is 

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@1c453e8). Click here to learn what that means.
The diff coverage is 92%.

@@            Coverage Diff             @@
##             develop    #1623   +/-   ##
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  Coverage           ?   66.58%           
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  Files              ?       78           
  Lines              ?    11200           
  Branches           ?     1572           
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  Hits               ?     7458           
  Misses             ?     3413           
  Partials           ?      329

Impacted Files	Coverage Δ
azurelinuxagent/ga/env.py	52.38% <0%> (ø)
...urelinuxagent/common/utils/extensionprocessutil.py	100% <100%> (ø)
azurelinuxagent/ga/exthandlers.py	82.04% <100%> (ø)
azurelinuxagent/common/cgroupconfigurator.py	91.35% <100%> (ø)
azurelinuxagent/common/cgroupapi.py	79.77% <87.8%> (ø)

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[larohra]

LGTM

[larohra]

Minor suggestion else LGTM