We have a lot of boiler plate template code like
```
{% if config.interface is defined and config.interface.remote_as is defined and config.interface.remote_as is not none %}
...
{% endif %}
```
This can be stripped down using a custom test to:
```
{% if config.interface.remote_as is vyos_defined %}
...
{% endif %}
```
In addition the new `vyos_defined` test supports comparison
```
{% if foo.bar.baz is vyos_defined('zoo') %}
...
{% endif %}
```
So the above will only evaluate to true if the variable `foo.bar.baz` is defined and its content is `zoo`
This is inspired from https://github.com/aristanetworks/ansible-avd/ which make heavy use of it.