Add switch to turn redundant log file off

[pmjdebruijn]

Description

Issue #1817 (this adds on option for the last part, without actually changing the default behavior)

Currently there is no way to disable logging to /var/log/waagent.log, which is redundant with the proper system log in journald.

This PR adds a configuration switch so it's easy to turn off logging to a file, and only have logging in journald.

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PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• Except for special cases involving multiple contributors, the PR is started from a fork of the main repository, not a branch.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made and Travis.CI is passing.

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.

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This change is 

[msftclas]

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.54%. Comparing base (10ae95b) to head (aa9a8f9).
⚠️ Report is 1074 commits behind head on develop.

Files with missing lines	Patch %	Lines
azurelinuxagent/agent.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1819      +/-   ##
===========================================
- Coverage    68.54%   68.54%   -0.01%     
===========================================
  Files           82       82              
  Lines        11729    11732       +3     
  Branches      1644     1645       +1     
===========================================
+ Hits          8040     8042       +2     
  Misses        3343     3343              
- Partials       346      347       +1     

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