Do not send localhost as the name for dhclient

[edwardsp]

The DHCP hostname is set to localhost.localdomain after running waagent -deprovision. This causes localhost to be added to Azure DNS and a call to nslookup localhost from somewhere on the VNET can get another VMs private IP address.

This PR has a proposed solution for this.

Description

Issue #2928

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PR information

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• New Unit tests were added for the changes made

Quality of Code and Contribution Guidelines

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[codecov]

Codecov Report

Merging #2930 (a42b4b8) into master (28345a5) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2930      +/-   ##
==========================================
- Coverage   71.97%   71.94%   -0.03%     
==========================================
  Files         104      104              
  Lines       15873    15879       +6     
  Branches     2273     2276       +3     
==========================================
  Hits        11424    11424              
- Misses       3926     3932       +6     
  Partials      523      523              

Files	Coverage Δ
azurelinuxagent/common/osutil/redhat.py	51.02% <0.00%> (-1.07%)	⬇️
azurelinuxagent/common/osutil/suse.py	44.68% <0.00%> (-0.98%)	⬇️
azurelinuxagent/common/osutil/default.py	59.79% <0.00%> (-0.13%)	⬇️