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

~~~
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

~~~
User credentials should not be in plaintext.
~~~

##### ACTUAL RESULTS

~~~
User credentials are in plaintext.
~~~

Comece pelo caminho de provisionamento do Remote Access VPN Service para roteadores virtuais e inspecione como /etc/ppp/chap-secrets é gerado. Determine a abordagem de autenticação compatível e suas implicações de segurança antes de alterar o comportamento. A tarefa estará concluída quando as credenciais de VPN não forem armazenadas em texto simples e a cobertura relevante de VPN verificar o resultado.

Escrita pelo modelo de indexação a partir do texto da issue.