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

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

~~~
Remote Access VPN Services (L2TP-over-IPsec)
~~~

##### CLOUDSTACK VERSION

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4.19, 4.20
~~~

##### 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

~~~
- 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.
~~~

從虛擬路由器的 Remote Access VPN Service 佈建流程著手，檢查 /etc/ppp/chap-secrets 的產生方式。在變更行為之前，確定受支援的驗證方式及其安全性影響。完成標準是 VPN 認證資訊不以純文字儲存，且相關的 VPN 覆蓋範圍能夠驗證結果。

由索引模型根據 Issue 內容生成。