TT#18360 Fix support of SKIP_STRICT_HOSTNAME_TEST/SKIP_DNS_CHECK_TEST after migration on YAML::XS

The values 'true'/'false' must be quoted into yml config nowadays, otherwise:

> root@debian-jessie-plain:~# cat /etc/ngcp-config/config.debian-jessie-plain.yml
> ---
> sipwise:
>   system_tester:
>     skip_dns_check_test: true
>     skip_strict_hostname_test: true
> root@debian-jessie-plain:~# ngcpcfg values sipwise.system_tester.skip_dns_check_test
> 1
> root@debian-jessie-plain:~# ngcpcfg values sipwise.system_tester.skip_strict_hostname_test
> 1

> root@debian-jessie-plain:~# cat /etc/ngcp-config/config.debian-jessie-plain.yml
> ---
> sipwise:
>   system_tester:
>     skip_dns_check_test: 'true'
>     skip_strict_hostname_test: 'true'
> root@debian-jessie-plain:~# ngcpcfg values sipwise.system_tester.skip_strict_hostname_test
> true
> root@debian-jessie-plain:~# ngcpcfg values sipwise.system_tester.skip_dns_check_test
> true

Change-Id: Idcbcfbeeda2cd39a7e44c278c3577c723a3636d1
(cherry picked from commit 461a288b49)

testrunner

@@ -39,8 +39,8 @@ if [ -n "${SKIP_DNS_CHECK_TEST}" ] || [ -n "${SKIP_STRICT_HOSTNAME_TEST}" ] ; th
 ---
 sipwise:
   system_tester:
-    skip_dns_check_test: ${SKIP_DNS_CHECK_TEST:-false}
-    skip_strict_hostname_test: ${SKIP_STRICT_HOSTNAME_TEST:-false}
+    skip_dns_check_test: '${SKIP_DNS_CHECK_TEST:-false}'
+    skip_strict_hostname_test: '${SKIP_STRICT_HOSTNAME_TEST:-false}'
 EOF
 else
   # remove the file if exist from the previous run