reverse-proxy: T6402: Fix invalid checks in validation script (backport #3523)

[mergify]

Change Summary

Fixed issues in reverse-proxy validation script where checks are not being run properly, due to invalid variable references.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T6402

Related PR(s)

Component(s) name

• load-balance -> reverse-proxy

Proposed changes

How to test

Smoketest result

vyos@vyos:~/vyos-1x$ python3 /usr/libexec/vyos/tests/smoke/cli/test_load-balancing_reverse-proxy.py
test_01_lb_reverse_proxy_domain (__main__.TestLoadBalancingReverseProxy.test_01_lb_reverse_proxy_domain) ... ok
test_02_lb_reverse_proxy_cert_not_exists (__main__.TestLoadBalancingReverseProxy.test_02_lb_reverse_proxy_cert_not_exists) ...
PKI does not contain any certificates!


Certificate "cert" not found in configuration!

ok
test_03_lb_reverse_proxy_ca_not_exists (__main__.TestLoadBalancingReverseProxy.test_03_lb_reverse_proxy_ca_not_exists) ... ok
test_04_lb_reverse_proxy_backend_ssl_no_verify (__main__.TestLoadBalancingReverseProxy.test_04_lb_reverse_proxy_backend_ssl_no_verify) ...
backend bk-01 cannot have both ssl options no-verify and ca-certificate
set!

ok
test_05_lb_reverse_proxy_backend_http_check (__main__.TestLoadBalancingReverseProxy.test_05_lb_reverse_proxy_backend_http_check) ... ok
test_06_lb_reverse_proxy_tcp_mode (__main__.TestLoadBalancingReverseProxy.test_06_lb_reverse_proxy_tcp_mode) ... ok
test_07_lb_reverse_proxy_http_response_headers (__main__.TestLoadBalancingReverseProxy.test_07_lb_reverse_proxy_http_response_headers) ...
service https_front must be set to http mode to use
http_response_headers!

ok

----------------------------------------------------------------------
Ran 7 tests in 30.531s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3523 done by [Mergify](https://mergify.com).