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

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• 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 

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[codecov]

Codecov Report

Merging #1819 into develop will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@             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     

Impacted Files	Coverage Δ
azurelinuxagent/agent.py	44.37% <50.00%> (-0.28%)	⬇️
azurelinuxagent/common/conf.py	79.06% <100.00%> (+0.24%)	⬆️

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