Hide VPN users' passwords in virtual router (use PAP instead of CHAP)

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

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Remote Access VPN Services (L2TP-over-IPsec)
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##### CLOUDSTACK VERSION

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4.19, 4.20
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##### CONFIGURATION

- Remote Access VPN Service enabled on guest networks or VPCs
- Create VPN Users

##### SUMMARY

The users' credentials for the VPN remote access service are in plaintext in /etc/ppp/chap-secrets file. Even though the VRs are secured and only the platform administrators have access to them, it is not a good practice and and may even have security consequences to leave the passwords in plain text.
It would be a good improvement if it were possible to hashing this data (or something like that).

##### STEPS TO REPRODUCE

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- Enable VPN remote access service in guest network or VPC
- Create a VPN User
- Verify that the /etc/ppp/chap-secrets file on the virtual router contains the VPN user credentials in plaintext.
~~~

##### EXPECTED RESULTS

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User credentials should not be in plaintext.
~~~

##### ACTUAL RESULTS

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User credentials are in plaintext.
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从虚拟路由器的 Remote Access VPN Service 配置流程入手，检查 /etc/ppp/chap-secrets 的生成方式。在更改行为之前，确定受支持的身份验证方式及其安全影响。完成标准是 VPN 凭据不以明文存储，并且相关的 VPN 覆盖范围能够验证结果。

由索引模型根据 Issue 内容生成。