hostname does not match /etc/hosts

[lguohan]

Description

After merging PR 1ef7403, there are lots of vsimage test failures.

https://sonic-jenkins.westus2.cloudapp.azure.com/job/vs/job/buildimage-vs-image-pr/2685/console
https://sonic-jenkins.westus2.cloudapp.azure.com/job/vs/job/buildimage-vs-image-pr/2686/
https://sonic-jenkins.westus2.cloudapp.azure.com/job/vs/job/buildimage-vs-image-pr/2687/console

TASK [saved original minigraph file in SONiC DUT(ignore errors when file doesnot exist)] ***
Friday 28 February 2020  04:13:41 +0000 (0:00:00.057)       0:00:16.348 ******* 
fatal: [vlab-01]: FAILED! => {"msg": "Timeout (12s) waiting for privilege escalation prompt: "}
...ignoring

TASK [create new minigraph file for SONiC device] ******************************
Friday 28 February 2020  04:13:53 +0000 (0:00:12.267)       0:00:28.615 ******* 
fatal: [vlab-01]: FAILED! => {"msg": "Timeout (12s) waiting for privilege escalation prompt: "}

It looks like the root cause the hostname is not changed, but /etc/hosts is changed.

root@sonic:/home/admin# cat /etc/hosts
127.0.0.1       localhost
127.0.0.1 vlab-01
root@sonic:/home/admin# cat /etc/hostname
sonic

I think this is related to your PR because I suspect this PR trigger some race condition that cause the hostname to be set back to sonic. As there is possibility to change the host OS hostname inside the container now.

Steps to reproduce the issue:
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Describe the results you received:

Describe the results you expected:

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

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[lguohan]

resolved by #4206