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)
~~~

##### 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.
~~~

仮想ルーター向けの Remote Access VPN Service のプロビジョニングパスから着手し、/etc/ppp/chap-secrets がどのように生成されるかを確認します。動作を変更する前に、サポートされている認証方式とそのセキュリティ上の影響を特定します。VPN の認証情報が平文で保存されず、関連する VPN のカバレッジで結果が検証されれば完了です。

索引モデルが issue の本文から書いたものです。