Ignore undefined values in customizable filter

When using the customizable filter to modify variables, if a template
overrides file is shared between multiple sets of images, it is sometimes
possible that a customised variable does not exist for a set of images.
A specific example:

My template overrides file has this:

{% set ironic_inspector_packages_append = ["python-pip"] %}

Normally I build binary/centos images.

kolla-build -t binary

Sometimes however I build a bifrost centos/source image, because bifrost
does not support binary images.

kolla-build -t source bifrost

The ironic inspector image does not define ironic_inspector_packages for
the source build, since it does not need any.

{% elif install_type == 'source' %}
    {% if base_distro in ['debian', 'ubuntu'] %}
        {% set ironic_inspector_packages = ['iptables'] %}
    {% endif %}

{{ macros.install_packages(ironic_inspector_packages |
        customizable("packages")) }}

The install_packages macro copes with the undefined variable, however
the customizable filter does not handle the base variable being
undefined if a customised example exists.

This change allows the customizable filter to ignore undefined
variables, even if they have been customised.

Change-Id: Ibe2fc91f5b6ceee8f937dd73c235128b2db525b1
Closes-Bug: #1809491

kolla/template/filters.py

@@ -13,10 +13,17 @@
 # limitations under the License.
 
 from jinja2 import contextfilter
+from jinja2 import Undefined
 
 
 @contextfilter
 def customizable(context, val_list, call_type):
+    # NOTE(mgoddard): Don't try to customise undefined values. There are cases
+    # where this might happen, for example using a generic template overrides
+    # file for building multiple image install types and/or distros, where
+    # variables are not defined in every case.
+    if isinstance(val_list, Undefined):
+        return val_list
     name = context['image_name'].replace("-", "_") + "_" + call_type + "_"
     if name + "override" in context:
         return context[name + "override"]