[AVS-lite] Use the configured password

[sreeiyer-1]

The default username / password was hard coded in the earlier versions. Instead, take it from the build system (rules/config).

DEFAULT_USERNAME & DEFAULT_PASSWORD are available as make variables. Access them in docker-sonic-alpinevs.mk. In the Dockerfile.j2, create the user using these when the switch comes up.

Unit tests :

1. Changed the password in rules/config
2. Dump the files in the switch to confirm that the user is created

root@docker-alpine-ctl:/home/admin#grep admin /etc/passwd
admin:x:1000:1000::/home/admin:/bin/bash
root@docker-alpine-ctl:/home/admin# grep admin /etc/shadow
admin:$6$5IjeFwZqOKTrn20f$Jmb2O0DnIqto9vW7ENvWQg0R86C23aC3EIte7NHlwTI6XSif.TWz7/KBlmJfjxNa1bUDZH1pXlXdyGQQwWaYB1:20693:0:99999:7:::
root@docker-alpine-ctl:/home/admin# 

[mssonicbld]

/azp run

[azure-pipelines]

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