🐞 NTP: Fix config template to init default parameters (sonic-net#18736)

fixes sonic-net#17906

#### Why I did it
To fix NTP config generation from the minigraph and save backward compatability

#### How I did it
Align `ntp.conf.j2` template to generate config out of empty `NTP_SERVER` DB configuration

#### How to verify it
Out of that NTP_SERVER configuration:
```json
{
  "10.210.25.32": {},
  "10.75.202.2": {}
}
```

The next config in `ntp.conf` file should be produced:
```
server 10.210.25.32
restrict 10.210.25.32 kod limited nomodify notrap noquery nopeer

server 10.75.202.2
restrict 10.75.202.2 kod limited nomodify notrap noquery nopeer
```

files/image_config/ntp/ntp.conf.j2

@@ -20,14 +20,14 @@ filegen clockstats file clockstats type day enable
 {# Adding NTP servers. We need to know if we have some pools, to set proper
 config -#}
 {% set ns = namespace(is_pools=false) %}
-{% for server in NTP_SERVER if NTP_SERVER[server].admin_state != 'disabled' and
-                               NTP_SERVER[server].resolve_as and
-                               NTP_SERVER[server].association_type -%}
+{% for server in NTP_SERVER if NTP_SERVER[server].admin_state != 'disabled' -%}
     {% set config = NTP_SERVER[server] -%}
     {# Server options -#}
     {% set soptions = '' -%}
-    {# Server access control options -#}
-    {% set aoptions = '' -%}
+
+    {# Define defaults if not defined -#}
+    {% set association_type = config.association_type | d('server') -%}
+    {% set resolve_as = config.resolve_as | d(server) -%}
 
     {# Authentication key -#}
     {% if global.authentication == 'enabled' -%}
@@ -49,17 +49,13 @@ config -#}
     {# Check if there are any pool configured. BTW it doesn't matter what was
     configured as "resolve_as" for pools. If they were configured with FQDN they
     must remain like that -#}
-    {% set config_as = config.resolve_as -%}
-    {% if config.association_type == 'pool' -%}
-        {% set ns.is_pools = true -%}
-        {% set config_as = server -%}
-    {% else -%}
-        {% set aoptions = aoptions ~ ' nopeer' -%}
+    {% if association_type == 'pool' -%}
+        {% set resolve_as = server -%}
     {% endif -%}
 
-{{ config.association_type }} {{ config_as }}{{ soptions }}
+{{ association_type }} {{ resolve_as }}{{ soptions }}
 {% if global.server_role == 'disabled' %}
-restrict {{ config_as }} kod limited nomodify notrap noquery{{ aoptions }}
+restrict {{ resolve_as }} kod limited nomodify noquery
 {% endif %}
 
 {% endfor -%}

src/sonic-config-engine/tests/sample_output/py3/ntp.conf

@@ -15,10 +15,10 @@ filegen peerstats file peerstats type day enable
 filegen clockstats file clockstats type day enable
 
 server 10.20.30.50 key 42 iburst version 3
-restrict 10.20.30.50 kod limited nomodify notrap noquery nopeer
+restrict 10.20.30.50 kod limited nomodify noquery
 
 pool pool.ntp.org iburst version 3
-restrict pool.ntp.org kod limited nomodify notrap noquery
+restrict pool.ntp.org kod limited nomodify noquery
 
 
 keys /etc/ntp.keys